Introduction

This book is the official companion documentation for the Library of Time: a collection of calendars and other ways humans measure time, each tied to a verifiable instant so dates can be compared and explored side by side.

The chapters follow the same organization as the site. They are presented here for educational and academic purposes.

Standard Time

Standard time systems are the fundamental time measurements we use in everyday life. These include basic time units from seconds to millennia, as well as local time and coordinated universal time (UTC).

Local Time

Overview

This is the current local time based on the timezone provided by your device or the timezone selected in the datetime selection.

Info

Local Time can vary depending on location, timezone, Daylight Savings Time, and historical adjustments.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

Local Time is sourced directly from JavaScript's Date library and your device's system time.

Calculation

This clock shouldn't need to be calculated, since dateTime already provides the local time.

currentDateTime.toTimeString();

UTC

Overview

Coordinated Universal Time is the global time standard. The time expressed here is the same regardless of timezone. It is based on the timezone of the Prime Meridian at 0°.

Info

UTC is the successor to GMT.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

UTC is sourced directly from JavaScript's Date library and your device's system time.

Calculation

This clock shouldn't need to be calculated, since dateTime already provides the UTC time.

currentDateTime.toUTCString();

Second

Overview

This is the fraction of time passed in the current second.

Info

This calculation is based on local time.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock cal be calculated by taking the current millisecond and dividing it by 1000.

const secondFraction = currentDateTime.getMilliseconds() / 1000;

Minute

Overview

This is the fraction of time passed in the current minute.

Info

This calculation is based on local time.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by adding the current fraction of a second to the current seconds and then dividing the total by 60.

const secondFraction = currentDateTime.getUTCMilliseconds() / 1000;
const minuteFraction = (currentDateTime.getUTCSeconds() +
                        secondFraction) / 60;

Hour

Overview

This is the fraction of time passed in the current hour.

Info

This calculation is based on local time.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by getting the minute fraction, adding that to the current minute, and dividing the total by 60.

const secondFraction = currentDateTime.getUTCMilliseconds() / 1000;
const minuteFraction = (currentDateTime.getUTCSeconds() +
                        secondFraction) / 60;
const hourFraction = (currentDateTime.getUTCMinutes() +
                        minuteFraction)/60;

Day

Overview

This is the fraction of time passed in the current day, based on the local timezone.

Info

This calculation is based on local time.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by adding the hour fraction to the current hour and then dividing the total by 24.

const secondFraction = currentDateTime.getUTCMilliseconds() / 1000;
const minuteFraction = (currentDateTime.getUTCSeconds() +
                        secondFraction) / 60;
const hourFraction = (currentDateTime.getUTCMinutes() +
                        minuteFraction) / 60;
const dayFraction = (currentDateTime.getUTCHours() +
                        hourFraction) / 24;

Month

Overview

This is the fraction of time passed in the current month.

Info

This calculation is based on local time.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by subtracting the start time of the current month from the start time of the next month, and dividing the total by 1000, 60, 60, and 24 to get the number of days in the current month.

const year = currentDateTime.getUTCFullYear();
const month = currentDateTime.getUTCMonth();
const nextMonth = createDateWithFixedYear(year, month + 1, 0);
const thisMonth = createDateWithFixedYear(year, month, 0);
const daysInMonth = (nextMonth - thisMonth)/1000/60/60/24;

Then add the current day fraction to the current day, and divide the total by the number of days in the current month.

const secondFraction = currentDateTime.getUTCMilliseconds() / 1000;
const minuteFraction = (currentDateTime.getUTCSeconds() +
                        secondFraction) / 60;
const hourFraction = (currentDateTime.getUTCMinutes() +
                        minuteFraction) / 60;
const dayFraction = (currentDateTime.getUTCHours() +
                        hourFraction) / 24;
// Subtract one from the current date to get number of passed days
const monthFraction = ((currentDateTime.getUTCDate() - 1) +
                        dayFraction) / daysInMonth;

Year

Overview

This is the fraction of time passed in the current year.

Info

Due to leap days, this fraction might not be a perfect 10x multiplication of the century calculation.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by getting the start of the current and next year.

const startOfYear = createDateWithFixedYear(year, 0, 1);
const startOfNextYear = createDateWithFixedYear(year + 1, 0, 1);

After that, the ratio between the difference of the current datetime and start of the year can be compared to the difference of the start of the next year and the start of the current year.

return (currentDateTime - startOfYear) / (startOfNextYear - startOfYear);

Decade

Overview

This is the fraction of time passed in the current decade.

Info

Due to leap days, midnight on the 1st of January of the 6th year in the decade might not be exactly 50% of the way through the decade.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by first getting the year number of the start of the current decade.

// Calculate ordinal decade start (ending in 1)
const year = currentDateTime.getUTCFullYear();
let startYear = Math.floor(year / 10) * 10 + 1;

This will provide the start of a decade beginning in the 1s place (2021, 2001, etc.). After that, the start of the current decade and the start of the next decade can be calculated.

const startThisDecade = createDateWithFixedYear(startYear, 0, 1);
const startNextDecade = createDateWithFixedYear(startYear + 10, 0, 1);

After that, the ratio between the difference of the current datetime and start of the decade can be compared to the difference of the start of the next decade and the start of the current decade.

return (currentDateTime - startThisDecade) /
    (startNextDecade - startThisDecade);

Century

Overview

This is the fraction of time passed in the current century.

Info

Due to leap days, midnight on the 1st of January of the 51st year in the century might not be exactly 50% of the way through the century.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by first getting the year number of the start of the current century.

// Calculate ordinal century start (ending in 1)
const year = currentDateTime.getUTCFullYear();
let startYear = Math.floor(year / 100) * 100 + 1;

This will provide the start of a century beginning in the 1s place (2201, 2001, etc.). After that, the start of the current century and the start of the next century can be calculated.

const startThisCentury = createDateWithFixedYear(startYear, 0, 1);
const startNextCentury = createDateWithFixedYear(startYear + 10, 0, 1);

After that, the ratio between the difference of the current datetime and start of the century can be compared to the difference of the start of the next century and the start of the current century.

return (currentDateTime - startThisCentury) /
    (startNextCentury - startThisCentury);

Millennium

Overview

This is the fraction of time passed in the current millennium.

Info

Due to leap days, midnight on the 1st of January of the 501st year in the millennium might not be exactly 50% of the way through the millennium.

Accuracy

This calculation is perfectly accurate to the millisecond.

Source

This is a simple calculation with no source.

Calculation

This clock can be calculated by first getting the year number of the start of the current millennium.

// Calculate ordinal millennium start (ending in 1)
const year = currentDateTime.getUTCFullYear();
let startYear = Math.floor(year / 1000) * 1000 + 1;

This will provide the start of a millennium beginning in the 1s place (3001, 2001, etc.). After that, the start of the current millennium and the start of the next millennium can be calculated.

const startThisMillennium = createDateWithFixedYear(startYear, 0, 1);
const startNextMillennium = createDateWithFixedYear(startYear + 10, 0, 1);

After that, the ratio between the difference of the current datetime and start of the millennium can be compared to the difference of the start of the next millennium and the start of the current millennium.

return (currentDateTime - startThisMillennium) /
    (startNextMillennium - startThisMillennium);

Computing Time

Computing Time is a collection of clocks and calendars that aren't typically used by any culture as a primary means of telling time. Instead, they are often converted into more colloquial forms of timekeeping.

Unix

Overview

Unix is the most widespread timing system in computing and on the internet. It is a simple count of number of seconds since midnight on January 1st, 1970. Many of the calculations on this website are derived from Unix timestamps.

Info

Unix time skips leap seconds, operating as if that time never happened.

Accuracy

As Unix is the source of all timekeeping systems on this site, it is perfectly accurate.

Source

Unix time is the source of all other timekeeping systems on this website. It is as accurate as JavaScript's Date library and your device's system time allow.

Some information for this clock came from this website.

Calculation

This clock shouldn't need to be calculated, since dateTime already provides the Unix time.

currentDateTime.getTime();

This will also automatically skip leap seconds.

GPS

Overview

GPS time is the standard by which all GPS satellites and GPS-enabled devices coordinate their positions. It is a simple count of seconds from midnight on January 6th, 1980. When converted into the Gregorian calendar, it drifts ahead by a second every now and then as it does not follow leap seconds found in other timekeeping standards.

Info

GPS became available to the public in 2000 CE.

Accuracy

This clock is considered to be perfectly accurate, as it's a simple calculation from Unix.

Source

Much of the information for this clock came from its Wikipedia article.

Some information for this clock came from this website.

Calculation

Calculating the current GPS time requires starting with the GPS epoch. and subtracting it from the current datetime.

gpsEpoch = new Date("1980-01-06T00:00:00Z").getTime();
// Calculate total time difference in seconds
let gpsTime = Math.floor((currentDateTime - gpsEpoch) / 1000);

After that, the leap seconds that have already passed need to be added.

const GPSleapSeconds = [
    "1981-06-30T23:59:59Z",
    "1982-06-30T23:59:59Z",
    "1983-06-30T23:59:59Z",
    "1985-06-30T23:59:59Z",
    "1987-12-31T23:59:59Z",
    "1989-12-31T23:59:59Z",
    "1990-12-31T23:59:59Z",
    "1992-06-30T23:59:59Z",
    "1993-06-30T23:59:59Z",
    "1994-06-30T23:59:59Z",
    "1995-12-31T23:59:59Z",
    "1997-06-30T23:59:59Z",
    "1998-12-31T23:59:59Z",
    "2005-12-31T23:59:59Z",
    "2008-12-31T23:59:59Z",
    "2012-06-30T23:59:59Z",
    "2015-06-30T23:59:59Z",
    "2016-12-31T23:59:59Z"
];

// Calculate how many leap seconds have occurred before the currentDateTime
let leapSecondsCount = 0;
GPSleapSeconds.forEach(leapSecond => {
    if (new Date(leapSecond).getTime() <= currentDateTime) {
        leapSecondsCount++;
    }
});

// Add leap seconds to account for the growing difference between GPS and UTC.
gpsTime += leapSecondsCount;

TAI

Overview

International Atomic Time is the average of several atomic clocks and is based on the passage of time on Earth's geoid. It is the basis for UTC but deviates from UTC by several seconds due to TAI not including leap seconds, specifically the number of leap seconds since 1972 plus 10 extra to account for missed leap seconds since 1958.

Info

Ironically, the clock displayed here is derived from UTC even though it is itself the basis for UTC.

Accuracy

This clock is considered to be perfectly accurate, as it's a simple calculation from UTC. However, this only applies to dates after the Unix epoch of 1 January 1970. Prior to that, leap seconds aren't taken into account in this calculation.

Source

Much of the information for this clock came from its Wikipedia article.

Some information for this clock came from this website.

Calculation

Calculating TAI is typically done using sensitive instruments rather than programmatically. However, an approximation that is good enough for an 'Exact' confidence classification on this site can be calculated by adding up the number of leap seconds that have passed plus 10 seconds.

const TAIleapSeconds = [
    "1972-06-30T23:59:59Z",
    "1972-12-31T23:59:59Z",
    "1973-12-31T23:59:59Z",
    "1974-12-31T23:59:59Z",
    "1975-12-31T23:59:59Z",
    "1976-12-31T23:59:59Z",
    "1977-12-31T23:59:59Z",
    "1978-12-31T23:59:59Z",
    "1979-12-31T23:59:59Z",
    "1981-06-30T23:59:59Z",
    "1982-06-30T23:59:59Z",
    "1983-06-30T23:59:59Z",
    "1985-06-30T23:59:59Z",
    "1987-12-31T23:59:59Z",
    "1989-12-31T23:59:59Z",
    "1990-12-31T23:59:59Z",
    "1992-06-30T23:59:59Z",
    "1993-06-30T23:59:59Z",
    "1994-06-30T23:59:59Z",
    "1995-12-31T23:59:59Z",
    "1997-06-30T23:59:59Z",
    "1998-12-31T23:59:59Z",
    "2005-12-31T23:59:59Z",
    "2008-12-31T23:59:59Z",
    "2012-06-30T23:59:59Z",
    "2015-06-30T23:59:59Z",
    "2016-12-31T23:59:59Z"
];

let leapSecondsCount = 0;
    TAIleapSeconds.forEach(leapSecond => {
        if (new Date(leapSecond).getTime() <= currentDateTime) {
            leapSecondsCount++;
        }
    });
    // Add accumulated leap seconds plus the initial 10
    taiDateTime.setSeconds(taiDateTime.getSeconds() + (10 + leapSecondsCount));

TT

Overview

Terrestrial Time is the timekeeping standard used by astronomers to calculate time as a global concept rather than in relation to the motion of the Earth. As the Earth's rotation slows due to tidal forces and earthquakes, UTC and TT will continue to drift apart. The difference between them is ΔT.

Info

TT is an ideal that can only ever be approximated, including at the precision of atomic clocks. The chosen epoch has no basis in science and is just used as a reference point for when ΔT was zero and thus TT and UTC were aligned.

Accuracy

This clock relies on the ΔT calculations, which are only approximations. On top of that, they were created in the 1990s are are certainly out of date by a few seconds.

Source

All of the information on this calendar came from its Wikipedia article.

Calculation

Calculating TT simply involves adding ΔT to the current datetime.

TT.setSeconds(currentDateTime.getSeconds() + getDeltaT(currentDateTime));

LORAN-C

Overview

Long Range Navigational time was the standard used by the US and other jurisdictions prior to the creation of GPS. It deviates from UTC by the number of leap seconds since 1972 and doesn't include the 10 extra leap seconds in TAI.

Info

LORAN-C uses a network of radio transmitters to determine distance using the synchronized time, similar to GPS that uses satellites.

Accuracy

It is difficult to find a current representation of LORAN-C despite it apparently still being in use. I have reconstructed this clock based off of the provided source as well as explanations of the specifics.

Source

Much of the information for this clock came from this website.

FILETIME

Overview

FILETIME is the timing method found on Windows filesystems. It is a simple count of number of nanoseconds since midnight on January 1st, 1601.

Info

Most systems use Unix or a similar epoch. FILETIME is unique in its choice of the year 1601.

Accuracy

FILETIME is accurate to the microsecond, but it does not count nanoseconds.

Source

All of the information on this calendar came from its Wikipedia article.

Julian Day Number

Overview

The Julian Day Number is a simple count of number of days since noon on 24 November 4713 BCE (1 January 4713 BC in the Julian Calerndar). The JDN is used by astronomers and programmers to simplify calculations for the passage of time, and many of the calculations in this website are based off of the JDN.

Info

There are many versions of the JDN, most of which involve truncating the large number for easier calculations.

Accuracy

This counter is rigorously-studied and exactly accurate, with the only question being the addition of Terrestrial Time.

Source

All of the information on this calendar came from its Wikipedia article.

Rata Die

Overview

Rata Die is similar to the Julian Day Number and is a simple count of number of days in the Gregorian Calendar since 1 January 1 CE.

Info

Rata Die was created as a way to calculate calendars more easily, though most day-based calculations on this site still use the Julian Day.

Accuracy

The Rata Die is a simple count of days, meaning it is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Julian Period

Overview

The Julian Period is a simple count of years in the Julian Calendar beginning at noon on 24 November 4713 BCE (1 January 4713 BC in the Julian Calerndar). It is used by historians to date events when no calendar date is given or when previous given dates are deemed to be incorrect.

Info

The Julian Period is the count of days since the last time Indiction, Solar and Lunar cycles all started on the same day, and repeats in a cycle of 7980 years.

Accuracy

The Julian Period is a simple count of days, meaning it should be exactly accurate. However, sources for this cycle aren't exactly clear if the year updates on the Julian or Astronomical calendar, so I have chosen to use the Julian Calendar. I've also taken some liberties by adding the cycle number, which is not explicitly defined in any source but is heavily implied.

Source

All of the information on this calendar came from its Wikipedia article.

Lilian Date

Overview

Lilian Date is a timekeeping standard similar to the Julian Day. It was invented by Bruce G. Ohms to be used with IBM systems and is named after Aloysius Lilius, the creator of the Gregorian calendar. It is a simple count of number of days since the beginning of the Gregorian calendar on October 15th, 1582 CE, which is Lilian 1.

Info

Lilian Date technically does not use a specific timezone for its calculation, so this website uses the Julian Day which is based on UTC.

Accuracy

The Lilian Date is a simple calculation on the Gregorian calendar, making it exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Ordinal Date

Overview

The Ordinal Date is a technique of describing the current date with just 5 digits. The first two digits are the year starting from 2000 CE, while the latter three digits is the simple day count for that year, with January 1 being 1.

The year doesn't necessarily have to be two digits and can become 3 or more if more than +- 99 years from 2000 CE.

Info

This counter used to be called the Julian Day, but that caused lots of confusion since the term had already been used to name the Julian Day Number, among other time formats.

Accuracy

The Ordinal Date is a simple calculation on the Gregorian calendar, making it exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

ISO 8601

Overview

ISO 8601 is the standard of displaying date and time provided by the International Organization for Standardization. It is based off the Gregorian calendar and utilizes year 0.

Info

ISO 8601 is intended to provide a clear, unambiguous date time format for international use.

Accuracy

ISO 8601 is derived directly from Unix time and thus is exactly accurate.

Source

ISO 8601 is actually a supported string of JavaScript's native Date library, so there is nothing for this website to calculate. General information came from its Wikipedia article.

ΔT

Overview

ΔT is an approximation of the difference in time between Terrestrial Time and UTC due to various factors that affect Earth's rotation, such as gravitational effects from other planets, earthquakes, and tidal forces. The two systems match around the year 1880 and deviate further away in time as a parabolic equation, with an uncertainty as much as two hours by the year 4000 BCE.

Info

The exact length of the day is slowly changing on the order of a few seconds per year. This rate is not constant, though it can be estimated.

Accuracy

ΔT is itself an approximation, so the results here can only be as good as that approximation. Unfortunately, there seems to be a bit of induced error on top of that, as my solutions don't exactly match those provided by Meeus. This could be due to JavaScript's base-2 calculations or due to a misunderstanding in some of the steps. However, they are very close, within a few seconds for any given output.

The epoch for this value is not an epoch but rather a moment where ΔT is close to zero.

Source

This calculation was sourced from the NASA Eclipse Web Site, which provides polynomial expressions for ΔT.

Calculation

Mars Sol Date

Overview

The Mars Sol Date, similar to the Julian Day Number, is the number of sols that have passed since the epoch. A sol is the name for the Martian day, and it is slightly longer than an Earth day. Currently I haven't been able to figure out exactly why the epoch was chosen. The day increments when the Airy-0 crater reaches midnight.

Info

One Mars sol is 39 minutes and 35 seconds longer than an Earth day.

Accuracy

This clock should be very accurate, though it relies on ΔT which ultimately is only an approximation.

Source

All of the information on this clock came from its Wikipedia article.

Julian Sol Number

Overview

The Julian Sol Number, created by Thomas Gangale, is similar to the Julian Day Number but it counts the number of sols that have passed since the epoch. A sol is the name for the Martian day, and it is slightly longer than an Earth day. This epoch marks an important Martian Vernal Equinox. The day increments when the Airy-0 crater reaches midnight.

In a chat I had with with Mr. Gangale, he expressed his desire for this standard to be deprecated, as the Mars Sol Date created by Michael Allison had received wider use. However, since it was used at one point, I have opted to include it in this website.

"The sooner that things become standardized, the better, so consider the JS to be obsolete." -Thomas Gangale, 2024

Info

One Mars sol is 39 minutes and 35 seconds longer than an Earth day.

Accuracy

This clock should be very accurate, though there are some very minor inaccuracies likely stemming from the redefiniton of the epoch of the Mars Sol Day, or perhaps slight differences in calculations of ΔT.

Source

Much of the information on this clock came from its Wikipedia article.

Dates can also be verified with this website, though some inaccuracies have been noted.

Julian Circad Number

Overview

The Julian Circad Number was a system created by Thomas Gangale in tandem with the Darian calendar for the Saturn moon, Titan. It is inspired by the Julian Day Number, but it counts circads from an epoch rather than days.

The epoch was chosen as the conjunction of the sun and Titan (when Titan was directly between the sun and Saturn) that occurred nearest to the epoch generally shared by the other Darian calendars.

Info

One circad is about 23 hours, 57 minutes and 13.11 seconds (0.998068439 days).

Accuracy

The accuracy of this timekeeping system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter in the Galilean calendars, but it isn't clear if it has also been accounted for in this clock.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Kali Ahargaṅa (IST)

Overview

Kali Ahargaṅa is a simple count of days since the kali epoch. According to Hindu timekeeping, the current yuga, Kali Yuga, began in 3101 BCE and will last for 432,000 years, ending in 428,899 CE.

Kali Yuga is the fourth, shortest, and worst of the four yugas.

Info

Each yuga has a shorter dawn and dusk period before and after the longer main period.

Accuracy

The Kali Ahargaṅa is based off the Gregorian calendar and is considered to be very accurate compared with historical records.

Source

Much of the information for this timekeeping system has come from its Wikipedia article.

You can find another converter for this system here.

Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number was created by Jean Meeus in 1998. It uses the New Moon of 6 January 2000 CE for its epoch, in this case denoted as Lunation 0.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Lunation Number is the standard used by this website to calculate other lunation numbers as well as certain calendars and calculations.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Brown Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number was created by Ernest William Brown. It uses the New Moon of 17 January 1923 CE for its epoch, in this case denoted as Lunation 1.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Brown Lunation Number can be calculated by adding 953 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Goldstine Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number was created by Herman Goldstine. It uses the New Moon of 11 January 1001 CE for its epoch, in this case denoted as Lunation 0.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Goldstine Lunation Number can be calculated by adding 37105 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Hebrew Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number is used by the Hebrew calendar. It uses the New Moon of 6 September 3761 BCE for its epoch, in this case denoted as Lunation 1.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Hebrew Lunation Number can be calculated by adding 71234 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Islamic Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number is used by the Hijri calendar. It uses the New Moon of 18 July 622 CE for its epoch, in this case denoted as Lunation 1.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Islamic Lunation Number can be calculated by adding 17038 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Thai Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number is used by the Buddhist calendar. It uses the New Moon of 22 March 638 CE for its epoch, in this case denoted as Lunation 0.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Thai Lunation Number can be calculated by adding 16843 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

More information can be found in its Wikipedia article.

Nabonassar Lunation Number

Overview

The Lunation Number is the number of New Moons since the epoch. There are several variations of this number.

This lunation number is associated with the Nabonassar Era of Babylon, using the new moon of 12 February 746 BCE as Lunation 1.

Info

As lunations are important to many calendars and cultures, there are many competing standards.

The Nabonassar Lunation Number can be calculated by adding 33963 to the Lunation Number.

Accuracy

The lunation number is a simple calculation of time since the epoch divided by the average lunar cycle length. As the lunar cycle can vary by several hours, the time that the lunation number changes might not exactly match the current lunation. It is mostly intended to be used as an approximate reference rather than as a rigid definition of when the lunation has occurred.

Source

This lunation number was calibrated using this converter.

Spreadsheet =NOW()

Overview

When using spreadsheets such as Excel or Google Sheets, =NOW() can be used to return the current date. This is saved in the background as a single floating point number, which can be revealed if the user incorrectly formats the date to a number.

Days are counted from the epoch of 30 December 1899 CE, which can also be seen if the user formats a zero to a date.

Info

One might notice that the epoch of 30 December 1899 CE is an odd choice. It is very close to 1 January 1900. Assuming that is day 1, then day 0 should be 31 December 1899.

The reason for this discrepancy is due to Microsoft Excel originally needing to be compatible with Lotus 1-2-3. The date system in Lotus 1-2-3 incorrectly determined the year 1900 to be a leap year (which was true in the Julian calendar but not the Gregorian). This error added an extra date of 29 February, which did not exist. Thus, in order for days after 28 February 1900 to be correct, the epoch must be shifted backwards by one day.

Accuracy

This calculation is a simple count based on a Gregorian date and is thus exactly accurate.

Source

All of the information on this clock came from its Wikipedia article.

Solar Calendars

Solar calendars are based on the Earth's revolution around the Sun, typically using the tropical year as their reference. These calendars often drift from their starting positions due to the lack of divisibility of the tropical year, by as many as 5 days per year but typically less than 1 day every 4 years. Some solar calendars are locked to a solstice or equinox and do not drift with regards to the equinoxes, though the equinoxes themselves have drift.

Gregorian

Overview

The Gregorian Calendar is a calculated solar calendar used by most of the world. It has 365 days, with an extra leap day every year divisible by 4 unless divisible by 100, except for years also divisible by 400.

The era is denoted 'CE' meaning 'Common Era' and 'BCE' meaning 'Before Common Era'. Dates can also be expressed in AD/BC as in the Julian calendar.

The calendar was issued by Pope Gregory XIII on October 15th, 1582 and is derived from the Julian Calendar after skipping 10 days between October 5th and 15th. The two calendars differ by the 400-year leap year rule.

This calendar is exactly accurate, however dates before October 15th 1582 are proleptic, and many countries did not adopt it until much later than 1582.

Info

After the initial 10-day skip in 1582 and the following centuries to the 21st century, the Gregorian calendar and the Julian calendar are 13 days apart.

Accuracy

The Gregorian calendar is exactly accurate, as it is what this entire site is based on. However, it does drift from the solar year ever so slightly at a rate of about 1 day every 3030 years without taking axial precession into account, or 1 day every 7700 years if taken into account.

Source

All of the information on this calendar came from its Wikipedia article.

Julian (UTC)

Overview

The Julian Calendar is a calculated solar calendar issued by Julius Caesar in 45 BC after several corrections to the solar date.

It features a leap day every 4 years, leading it to drift from the Gregorian calendar by 3 days every 400 years. Years are denoted 'AD' or 'Anno Domini', meaning 'in the year of the Lord', as well as 'BC' meaning 'Before Christ'.

The Julian calendar was the principal calendar in much of the world, especially Europe, prior to the adoption of the Gregorian calendar.

Info

The Julian calendar drifts from the solar year by about 1 day every 129 years, as it has too many leap years. The Gregorian calendar is meant to correct this drift. As of the 21st century, the two calendars are 13 days apart.

Accuracy

The Julian calendar is exactly accurate in relation to the Gregorian calendar, but dates before 40 BC might not reflect civic dates of the era due to a series of corrections.

The date of leap days might not be exactly aligned with the Gregorian calendar here, but they are accurate to the year.

Source

All of the information on this calendar came from its Wikipedia article.

Astronomical (UTC)

Overview

The Astronomical calendar is a standard for calculating dates of astronomical or historical events. It is a combination of the Gregorian calendar for dates after midnight on 15 October 1582 and the Julian calendar for dates before.

Unlike the Julian calendar, it utilizes Year 0 to enable the easier calculation of years, which causes it to appear 1 year ahead of the Julian calendar before 1 AD.

Years are not denoted with anything and are simply listed as a positive or negative number.

Info

Accuracy

This calendar is a simple calculation and is considered to be exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

French Republican (CET)

Overview

The French Republican calendar was an observational solar calendar used during and after the French Revolution from 1793 to 1805.

It featured twelve months of 30 days each, broken into 3 weeks of 10 days. The remaining 5 or 6 days of each solar year were the Sansculottides, to be treated as national holidays at the end of the year.

The new year started on the Autumn Equinox, and years were written in Roman numerals with the era name of 'l'ère républicaine', or 'Republican Era', abbreviated here as 'RE'.

Info

Accuracy

This calendar depends on the accuracy of the equinox calculations, which are generally within a few minutes of accuracy and should only affect years where the equinox happens very close to midnight CET.

The French Republican calendar also had some issues with rules that contracticted each other, like a 4 year leap year rule conflicting with the date of the equinox. That rule has been ignored in favor of the equinox rule.

Source

All of the information on this calendar came from its Wikipedia article.

Era Fascista (CET)

Overview

Era Fascista is a simple count of number of years since the start of the Fascist Era in Italy on October 29th, 1922, starting with Anno I.

Taking inspiration from the French Republican calendar, years were written in Roman numerals and it was intended to replace the Gregorian calendar.

Info

Era Fascista didn't really implement months, as it was used alongside the Gregorian calendar. Enscriptions marked in Era Fascista dates could use a number of different abbreviations, such as 'Anno', 'E.F.', 'Anno Fascista', 'A.F.', or simply 'A.'.

Accuracy

Era Fascista is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Coptic (EET)

Overview

The Coptic calendar, also known as the Alexandrian calendar, was a calculated solar calendar used in Egypt until the adoption of the Gregorian calendar in 1875. It is based on the ancient Egyptian calendar but with leap years every four years, keeping it in sync with the Julian calendar while sharing months and days with the Ge'ez calendar.

It has 12 months of 30 days plus a smaller 13th month of 5 or 6 days. The new year starts on the 11th or 12th of September, and years are abbreviated with 'AM', meaning Anno Martyrum, or 'Year of the Martyrs'.

The Coptic calendar is still in use today by Egyptian farmers as well as the Coptic Orthodox Church.

Info

The Ge'ez calendar is precisely aligned with the Coptic calendar for its months and days. It's epoch, translated to 'Year of the Martyrs', is counted from the year Diocletian became Emperor of Rome, which was followed by a period of mass persecution of Christians.

Accuracy

The Coptic calendar is intrinsically based on and locked to the Julian calendar, making it perfectly accurate.

Source

Much of the information on this calendar came from its Wikipedia article.

This calendar has been calibrated using the calendar found here.

Ge'ez (EAT)

Overview

The Ge'ez calendar is the official calendar of Ethiopia. A calculated solar calendar, it has 12 months of 30 days plus a smaller 13th month of 5 or 6 days. It has a leap day every 4 years, keeping it in sync with the Julian calendar while sharing months and days with the Coptic calendar.

The New Year starts on September 11th or 12th, with years abbreviated with ዓ.ም. which is pronounced 'am', short for Amätä Mihret, meaning 'Year of Mercy'.

Info

The Ge'ez calendar is precisely aligned with the Coptic calendar for its months and days. It nearly shares an epoch with the Julian calendar, as they both are counting years since the same event, but is actually 7-8 years behind due to a difference in calculation of the date of the Annunciation.

Accuracy

The Ge'ez calendar is intrinsically based on and locked to the Julian calendar, making it perfectly accurate.

Source

Much of the information on this calendar came from its Wikipedia article.

This calendar has been calibrated using the calendar found here.

Minguo (CST)

Overview

The Minguo calendar, also known as the Republic of China calendar, is a Gregorian-based calendar used in Taiwan and its territories. Following the traditional convention of numbering years after the current dynastic era, dates are counted in 民國 ('MinGuo'), translated as 'Year of the Republic' with year 1 being the establishment of the ROC in 1912, while its numerical months (月 "yue") and days (日 "ri") follow the Gregorian calendar.

The Minguo calendar was also used in China between 1912 and the fleeing of the ROC to Taiwan in 1949.

Info

Prior to the establishment of the Republic of China in 1912, the Chinese people and government used the traditional Chinese lunisolar calendar. The ROC adopted the Gregorian calendar for official business but used the new Era of the Republic as the epoch. After claiming victory over the Chinese mainland in 1949, the PRC opted to use the Gregorian calendar along with its epoch.

Accuracy

The Minguo calendar is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Thai (THB)

Overview

The Thai solar calendar is a Gregorian-based calendar used in Thailand. It represents the number of years of the current Buddhist Era (B.E.), the Era of the Shaka Buddha. Year 0 falls on the Gregorian year of 543 BCE.

Info

Accuracy

The Thai solar calendar is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Juche (KST)

Overview

The Juche calendar was a Gregorian-based calendar used in North Korea. It represented the number of years since the birth year of Kim Il Sung, the founder of the DPRK, and was used from 1997 to 2024.

Info

Juche refers to the specific ideology of the Worker's Party of Korea and is related to Marxist-Leninism.

Accuracy

The Juche calendar is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Byzantine (TRT)

Overview

The Byzantine calendar was the official Julian-based calendar of the Byzantine Empire from 988 to 1453 and was used in Ukraine and Russia until 1700.

It differed from the Julian calendar with the new year starting on September 1st and the epoch being September 1st, 5509 BC (July 19th, 5508 BCE in the proleptic Gregorian calendar). Years are counted in AM, or 'Anno Mundi' meaning 'Year After Creation'.

Info

Accuracy

The Byzantine calendar is intrinsically based on and locked to the Julian calendar, making it perfectly accurate. However, there is some confusion as to when the day actually begins, with sources seemingly contradicting each other.

Source

All of the information on this calendar came from its Wikipedia article.

Florentine (CET)

Overview

The Florentine calendar was a Julian-based calendar used in the Republic of Florence during the Middle Ages. It differed from the Julian calendar with the new year starting on March 25th.

This meant that January 1st of a given year was immediately after December 31st of the same year, and March 24th of that year was followed by March 25th of the next year.

Days also started at sunset (of the previous calendar day on the Julian calendar), which is approximated here as 6:00pm in Florence.

Info

Accuracy

The Florentine calendar is intrinsically based on and locked to the Julian calendar, making it perfectly accurate. The only inaccuracies are the differences in the approximation of sunset to the actual time of sunset, which is expected to only differ by a few hours or minutes per day.

Source

All of the information on this calendar came from its Wikipedia article.

Pisan (CET)

Overview

The Pisan calendar was a Julian-based calendar used in the Republic of Pisa from the Middle Ages until the adoption of the Gregorian calendar in the 18th century.

It differs from the Julian calendars with years beginning on March 25th, meaning March 24th of one year is followed by March 25th of the next year. The Pisan calendar runs ahead of the Julian calendar, with the two calendars sharing dates only during January, February, and the beginning of March before the Pisan calendar year increments. It is one year ahead of the Florentine calendar.

Info

Accuracy

This calendar is based on the Julian calendar and is considered to be exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Venetian (CET)

Overview

The Venetian calendar was a Julian-based calendar used in the Republic of Venice from the Middle Ages until the adoption of the Gregorian calendar in the 18th century.

It differed from the Julian calendar with years beginning on March 1st. The Venetian year runs behind the Julian calendar during the months of January and February before syncronizing for the remaining months.

Info

Accuracy

This calendar is based on the Julian calendar and is considered to be exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Baháʼí (IRST)

Overview

The Baháʼí calendar is an observational solar calendar of the Baháʼí Faith. It begins its New Year on the day of the Spring Equinox, preventing it from drifting from the tropical year and causing it to very slowly drift through the Gregorian year following the precession of the equinoxes.

It features 19 months (or sometimes referred to as weeks) of 19 days, for a total of 361 days. The remaining 4 or 5 days of each year are called Ayyám-i-Há and take place between the final two months, Mulk and 'Alá', typically at the end of February.

Days start at sunset in Tehran, which is approximated here as 18:00 IRST. Years are denoted with 'BE', meaning Baháʼí Era.

Info

The new year starts on the day that the equinox occurs before noon in Iran, or the next day if it occurs after noon. The starting of the new year results in the final month having 29 or 30 days depending on when exactly the equinox occurs.

Accuracy

The accuracy of this calendar depends on the equinox calculations and may be off by a day for a whole year especially if it happens very close to sunset, but it is likely to self-correct by the next year. The equation breaks down considerably if rolled back or forward several thousand years as the equinox drifts due to precession and Terrestrial Time invokes inaccuracies.

Dates may change slightly too early or late depending on the real time of sunset.

There seems to be an issue with the stated epoch on Wikipedia being 1 day later than the calculated epoch here.

Source

A lot of the information about this calendar came from its Wikipedia article.

Dates can be referenced at the official Baháʼí website.

One of the people running the live chat at Baha'is of the United States was kind enough to provide me with their 50-year calendar, which has been used to calibrate this calendar.

Pataphysical

Overview

The Pataphysical calendar is a Gregorian-derived calendar. It is based off of the philosophy of Pataphysics, which is a parody of science created by Alfred Jarry in 1893, though the calendar wasn't created until 1949.

It features 13 months of 29 days, though for each month the 29th day is imaginary except for the month of Gidouille as well as Gueules in leap years. New Year is on September 8th of the Gregorian calendar, and the epoch, denoted A.P., is the day of Alfred Jarry's birth on 8 September 1873 CE.

Info

Accuracy

The Pataphysical calendar is based off the Gregorian calendar and is thus exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Discordian

Overview

The Discordian calendar is a Gregorian-derived calendar used in the virtual religion of Discordianism. It features 5 months, each of 73 days, with the year beginning on January 1st.

It follows Gregorian leap years, inserting a day between the 59th and 60th of the month of Chaos, lining up with February 29th on the Gregorian calendar. The leap day is called 'St. Tib's Day', and it takes place outside of any month or week, as though the calendar paused for a day.

Years are denoted with 'YOLD' meaning 'Year of Our Lady Discord'.

Info

Accuracy

The Discordian calendar is based off the Gregorian calendar and is thus exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

SCA

Overview

The SCA calendar is a Gregorian-based calendar used by the Society for Creative Anachronism.

Years are written as roman numerals and denoted with 'A.S.', short for 'Anno Societatis', meaning 'in the year of the Society', counting the number of years since the First Tournament on 1 May 1966 CE. Years begin on May 1st, so April 30th is the last day of the previous year.

Info

Accuracy

As this calendar is based off the Gregorian calendar, this calculation is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article as well as the SCA website.

Solar Hijri (IRST)

Overview

The Solar Hijri calendar is an observational solar calendar of Islam. It is the official calendar of Iran and Afghanistan. Each year begins on the Spring Equinox or the day after; thus it has no intrinsic error and it very slowly drifts through the Gregorian year following the precession of the equinoxes.

It features 12 months, corresponding to the zodiacal signs, with the first 6 having 31 days and the latter 6 having 30 (or 29) days to account for the sun traveling slower through the Zodiac due to Earth's oblong orbit. In Afghanistan the month names still refer to the Zodiac while elsewhere this calendar uses the Zoroastrian month names.

Unlike the lunar Hijri calendar, days start at midnight, though they both share the same epoch of number of years from the Hijrah. In this calendar, years are denoted with 'SH', 'HS', 'AH', or 'AHSh', typically referencing the name of the calendar.

Info

The new year starts on the day that the equinox occurs before noon in Iran, or the next day if it occurs after noon. The starting of the new year results in the final month having 29 or 30 days depending on when exactly the equinox occurs.

Accuracy

This calendar is reasonably accurate for modern years, but as its calculation relies on the calculation of the equinox, it may experience significant errors for years that are thousands of years out from modern times. It also approximates sunset in Tehran.

This calendar also may experience errors in its alignment with the Zodiac, as it is tied to the precession of the equinoxes.

Source

Much of the information on this calendar came from its Wikipedia article.

Qadimi (IRST)

Overview

The Qadimi calendar is a calculated solar calendar of Zoroastrianism. It was intended to follow the Spring Equinox, but a lack of intercalary days has resulted in this calendar drifting significantly.

It features 12 months of 30 days, plus a period of 5 days at the end of each year called the Gatha days. Each of the 30 days of the month are named, as well as each of the Gatha days. Days start at sunrise in Iran.

Years are denoted with 'Y.Z.' for the 'Yazdegerdi era', a count of years since the accession of the last Sassanid ruler, Yazdegerd III, but there have been several epochs used in the past.

Compared to the Gregorian calendar, the Qadimi calendar drifts by about 1 day every 3 years.

Info

Accuracy

This calendar is a simple calculation based off the Gregorian calendar. However, historically this calendar has received many revisions, particularly prior to 1006 CE, so the dates here might not accurately reflect historical dates.

Source

Much of the information on this calendar came from its Wikipedia article.

This calendar can be calibrated using the calculator at this site.

Egyptian Civil (EET)

Overview

The Egyptian Civil calendar was a calculated solar calendar used by Ancient Egypt, alongside its lunar calendar.

It featured 12 months of 30 days, divided into 3 seasons of 4 months each. The months have names but are usually labeled by their sequence in each season, leading to a pattern of [month] [season] [day]. Each year has 5 intercalary days with individual names at the end, for a total of 365 days.

The new year historically was intended to mark the heliacal rising of the star Sirius, but due to its inaccuracy it drifted by one day every 3 years. The new year eventually lines back up with the heliacal rising of Sirius every 1461 years, called the Sothic Cycle.

The epoch changed with each dynasty, and I could not find evidence of a standardized epoch. Here I have chosen to show the years since the believed beginning of the calendar, which is the day of the heliacal rising of Sirius in 2781 BCE.

Info

The intercalary days, called Heriu Renpet, celebrated the birthdays of the children of the god Nut.

Accuracy

This calendar is based on the Sothic Cycle, which is well-established to the Gregorian calendar via the Julian calendar. Some liberties were taken with the epoch, which is why it is in parentheses.

Source

Much of the information on this calendar came from its Wikipedia article.

Month names and other general data came from this site.

ISO Week Date

Overview

The ISO Week Date is part of the ISO standard for time. It is Gregorian-derived and breaks the year entirely into a whole number of weeks rather than months, which is often useful for business accounting.

Each year can have either 52 weeks (364 days) or 53 weeks (371 days).

Info

The ISO Week Date follows the format of Year-Week-Weekday, with Monday being the first day of the week and Sunday being the 7th day of the week.

Accuracy

Being an ISO standard with a standard calculation, this calendar is expected to be perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Haab (CST)

Overview

The Haab is a calculated solar calendar and one of many calendars used by the Maya people. It features 18 months of 20 days plus a short month of 5 days, with days counted starting from 0. Thus, it has 365 days with no intercalation.

The Haab combines with another Maya calendar, the 260-day Tzolkin, to create the Maya Calendar Round. This is the cycle that the two calendars create, which takes roughly 52 years to complete.

Years do not increment outside the Calendar Round, so it is impossible to say for certain what the epoch for the Haab should be, but sometime around 550 BCE is accepted among historians.

Info

Accuracy

This calendar is still used today in some Maya groups, and it has been calibrated using the calculator provided by the Smithsonian National Museum of the American Indian. However, it also relies on the Long Count calendar being accurate.

Source

Much of the information on this calendar can be found at its Wikipedia article.

The Smithsonian website has the current day as well as a converter, though it is broken for dates before the Long Count epoch.

Anno Lucis

Overview

Anno Lucis is a Gregorian-based year numbering system used by the Freemasons.

Years are denoted with 'AL', which stands for Anno Lucis, meaning "in the Year of Light". This refers to the proposed year of creation from the Hebrew calendar.

Info

Accuracy

As this is a simple addition to the Gregorian calendar, this calculation is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Tabot (EST)

Overview

The Tabot calendar, an anacronym of "The Anointed Body of Testimony", is a Gregorian-derived calendar of Rastafarianism. It features 12 months of 30 days, except for the final month of 35 days and an extra leap day added onto the 4th month.

The epoch is the coronation of H.I.M. Haile Selassie 1st as Emperor of Abyssinia on 2 November 1930 CE. Years are denoted with 'H.I.M', meaning 'His Imperial Majesty'.

New Year's Day is always on 2 November in the Gregorian calendar, and leap years occur in tandem with the Gregorian calendar.

Info

Accuracy

As this calendar is intrinsically locked to the Gregorian calendar, this calculation is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Icelandic (UTC)

Overview

The Icelandic calendar is the traditional calculated solar calendar of Iceland. It features two seasons, Summer and Winter, called 'misseri' meaning 'semester', each with 26-28 weeks. There is no epoch, and years do not necessarily begin with either misseri.

Although the misseri and weeks are the primary units, the misseri are further broken down into 6 months each, all of which consist of 30 days for a total of 360 days. Another four days are added between the 3rd and 4th months of Summer (midsummer) called 'Aukanætur' and is treated as a holiday. This brings the year to exactly 52 weeks or 364 days.

In order to keep the calendar aligned with the solar year, an intercalary week is added every 7 years, called 'Sumarauki' and occurring immediately after Aukanætur. Modern intercalary rules ensure the start of Summer always happens on the Thursday that occurs between 19-25th April of the Gregorian calendar (and previously 9-15th of the Julian calendar.)

Info

Because of the 4 days of Aukanætur, Summer is slightly longer than Winter, and neither is divisible cleanly by the 7-day week. The beginning of Winter is not on the same day of the week as the beginning of Summer. Instead, the beginning of the week shifts depending on the current misseri, resulting in the final week of each misseri being incomplete.

Accuracy

This calendar has a long history with several slight changes, especially regarding modern/old names and specific starting and ending times of misseri and weeks. I am reasonably sure it is accurate to official sources, but the information is somewhat fragmented.

Source

All of the information for this calendar came from this paper.

Saka Samvat (IST)

Overview

The Śaka Samvat, also known as the Indian National Calendar, is a Gregorian-derived calendar used for official government communications and media within India.

This calendar has 12 months of 30 or 31 days, with the second through fifth months having 31 days and the latter 6 having 30 to account for the yearly variation in Earth's orbital speed. Leap years follow the Gregorian calendar, lengthening the first month from 30 to 31 days.

Years are not denoted with anything, being shown as a simple number counting the number of years of the Shaka Era beginning on the day of the Northern Equinox in 78 CE.

As it follows the intercalation rules of the Gregorian calendar, its months start on the same day of the Gregorian calendar every year except for the month of Chaitra which starts one day earlier during leap years.

Info

Accuracy

As this calendar is intrinsically locked to the Gregorian calendar, this calculation is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article, but there are some additional details found here.

Other Calendars

Other calendars include specialized, ancient, and unique timekeeping systems that don't fit into the standard solar, lunar, or lunisolar categories. These include Maya calendars, space-based calendars for other planets and moons, and various cultural and historical calendar systems.

Maya Long Count (CST)

Overview

The Maya Long Count calendar is essentially a simple count of the number of days since the Maya date of creation. It is a five digit number, typically expressed with periods between the digits, made up of base-20 counters with the exception of the middle-right digit which is base-18.

Starting with the right, the smallest unit is the kʼin, which is equivalent to a day. Twenty kʼins make up one winal, 18 winals make up one tun, 20 tuns make up one kʼatun, and finally 20 kʼatuns make up one bʼakʼtun. A bʼakʼtun is roughly 394 solar years.

The Maya Long Count Calendar was of international interest in 2012 as it was the time when the bʼakʼtun incremented from 12 to 13, leading to superstitious theories and hysteria.

Info

Notably, winals are counted in base-18 rather than base-20 like the rest of the units. This is to reasonably match the tun to the length of the solar year. However, it is still over 5 days short, meaning it will drift about half as much as a true lunar calendar. 20 winals would be 400 days, which wouldn't have been as useful.

Accuracy

Correlating the Maya Long Count calendar was a matter of debate even in recent times. The majority of scholars seem to have accepted the Goodman–Martinez–Thompson (GMT) correlation.

With that in mind, this calendar is actually very easy to calculate, as it is just a count of days since the epoch, not unlike the Julian Day Number. It has no concept of intercalary time such as leap days and the count is agnostic of the solar or lunar years. The only method of inaccuracy with this calendar could be when exactly each day increments, but that does not affect the rest of the calendar in any meaningful way.

Source

Much of the information on this calendar can be found at its Wikipedia article.

The Smithsonian website has the current day as well as a converter, though it is broken for dates before the epoch.

Tzolkin (CST)

Overview

The Tzolkin is one of the calendars used by the Maya. It is a 260-day calendar with an unknown significance; suggestions include simple multiplications of important numbers as well as the length of a typical human gestation period.

The Tzolkin is a unique calendar, with day numbers increasing sequentially from 1-13 while the 20 day names also increment, leading to a system where 1 of a given day name is followed by 2 of the next day name, completing a cycle in 260 days.

The Tzolkin combines with another Maya calendar, the 365-day Haab, to create the Maya Calendar Round. This is the cycle that the two calendars create, which takes roughly 52 years to complete.

Cycles do not increment outside the Calendar Round, so it is impossible to say for certain what the epoch for the Tzolkin should be, but sometime around 1100 BCE is accepted among historians.

Info

Accuracy

This calendar is still used today in some Maya groups, and it has been calibrated using the calculator provided by the Smithsonian National Museum of the American Indian. However, it also relies on the Long Count calendar being accurate.

Source

Much of the information on this calendar can be found at its Wikipedia article.

The Smithsonian website has the current day as well as a converter, though it is broken for dates before the Long Count epoch.

Lord of the Night | Y (CST)

Overview

The Lord of the Night is a day cycle used by the Maya people. It is a simple cycle of 9 incrementing days that reflect the Maya deity that rules that night.

The names of 8 of the lords have not been identified, but the G9 lord is named Pauahtun.

The Maya also had another day cycle of 7 days of unknown utility, referred to simply as 'Y'.

Info

The Lords of the Night were also known by the Aztecs, and historians have identified the names of all 9 deities.

Accuracy

Being a simple calculation, these two cycles are expected to be perfectly accurate.

Source

Much of the information on this calendar can be found at its Wikipedia article.

The Smithsonian website has the current day as well as a converter, though it is broken for dates before the Long Count epoch.

Darian (Mars)

Overview

The Darian calendar is a proposed calculated solar calendar for use on Mars. It was created in 1985 by Thomas Gangale and named after his son, Darius.

It takes the ~668.5 sol Martian year (~687 Earth days) and divides it into 24 months of 28 or 27 sols. The new year is on the day of the Martian Northern Equinox.

The epoch is the Vernal Equinox of Julian Sol Number 0, taking place on 12 March 1609 CE at 18:40:06 UTC.

Leap years add one extra day in the final month, and they take place if the year number is odd or divisible by 10, unless also divisible by 100 except if divisible by 1000, though this formula changes after Darian year 2000 and gets updated every few thousand years as the orbit of Mars becomes more eccentric.

Info

Accuracy

This calendar depends on the Julian Sol Number which is in turn based on the Mars Sol Date. Assuming these are all accurate, then the Darian calendar should be correct.

There is one more stipulation that the current calendar is only perfectly accurate between the years 0 and 2000 due to the shortening of the Martian equinox year. Whether those are Martian years or Earth years isn't clear, but the difference is rather small and is currently ignored for this calendar.

Source

Much of the information on this calendar can be found at its Wikipedia article.

The actual creator of the calendar has a website and a date converter here, but it uses a slightly different Terrestrial Time correction for the Mars sol.

Galilean (Io)

Overview

The Galilean calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Earth-based Gregorian calendar, roughly sharing an epoch and most month names.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. There are 13 months, with the 'extra' month of Mercedonius added between the February and March analogs. Generally months have 32 circads, but Ganymede's Junius is only 24 circads, as well as the month of December for all moons in non-leap years. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean calendars use roughly the same epoch, within a week, as the Gregorian calendar, each of which corresponds with their Meridian Time.

The Galilean months are roughly equal to the Gregorian months, though with slightly different names as well as a 13th month added between Februarius and Martius. Each moon has a slightly different day arrangement.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

I was unable to properly understand the intercalary system employed by Mr. Gangale, so I introduced my own while attempting to match his intent as closely as possible.

The name of this calendar was only implied in the original text but never explicitly stated.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Galilean (Europa)

Overview

The Galilean calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Earth-based Gregorian calendar, roughly sharing an epoch and most month names.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. There are 13 months, with the 'extra' month of Mercedonius added between the February and March analogs. Generally months have 32 circads, but Ganymede's Junius is only 24 circads, as well as the month of December for all moons in non-leap years. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean calendars use roughly the same epoch, within a week, as the Gregorian calendar, each of which corresponds with their Meridian Time.

The Galilean months are roughly equal to the Gregorian months, though with slightly different names as well as a 13th month added between Februarius and Martius. Each moon has a slightly different day arrangement.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

I was unable to properly understand the intercalary system employed by Mr. Gangale, so I introduced my own while attempting to match his intent as closely as possible.

The name of this calendar was only implied in the original text but never explicitly stated.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Galilean (Ganymede)

Overview

The Galilean calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Earth-based Gregorian calendar, roughly sharing an epoch and most month names.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. There are 13 months, with the 'extra' month of Mercedonius added between the February and March analogs. Generally months have 32 circads, but Ganymede's Junius is only 24 circads, as well as the month of December for all moons in non-leap years. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean calendars use roughly the same epoch, within a week, as the Gregorian calendar, each of which corresponds with their Meridian Time.

The Galilean months are roughly equal to the Gregorian months, though with slightly different names as well as a 13th month added between Februarius and Martius. Each moon has a slightly different day arrangement.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

I was unable to properly understand the intercalary system employed by Mr. Gangale, so I introduced my own while attempting to match his intent as closely as possible.

The name of this calendar was only implied in the original text but never explicitly stated.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Galilean (Callisto)

Overview

The Galilean calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Earth-based Gregorian calendar, roughly sharing an epoch and most month names.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. There are 13 months, with the 'extra' month of Mercedonius added between the February and March analogs. Generally months have 32 circads, but Ganymede's Junius is only 24 circads, as well as the month of December for all moons in non-leap years. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean calendars use roughly the same epoch, within a week, as the Gregorian calendar, each of which corresponds with their Meridian Time.

The Galilean months are roughly equal to the Gregorian months, though with slightly different names as well as a 13th month added between Februarius and Martius. Each moon has a slightly different day arrangement.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

I was unable to properly understand the intercalary system employed by Mr. Gangale, so I introduced my own while attempting to match his intent as closely as possible.

The name of this calendar was only implied in the original text but never explicitly stated.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Darian (Io)

Overview

The Galilean Darian calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Mars-based Darian calendar, roughly sharing an epoch as well as all 24 months.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. Generally months have 32 circads, with few exceptions, most notably the final month that can be intercalated to have 24, 32, or 40 circads. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean Darian calendars use roughly the same epoch, within a week, as the Martian Darian calendar, each of which corresponds with their Meridian Time.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Darian (Europa)

Overview

The Galilean Darian calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Mars-based Darian calendar, roughly sharing an epoch as well as all 24 months.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. Generally months have 32 circads, with few exceptions, most notably the final month that can be intercalated to have 24, 32, or 40 circads. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean Darian calendars use roughly the same epoch, within a week, as the Martian Darian calendar, each of which corresponds with their Meridian Time.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Darian (Ganymede)

Overview

The Galilean Darian calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Mars-based Darian calendar, roughly sharing an epoch as well as all 24 months.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. Generally months have 32 circads, with few exceptions, most notably the final month that can be intercalated to have 24, 32, or 40 circads. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean Darian calendars use roughly the same epoch, within a week, as the Martian Darian calendar, each of which corresponds with their Meridian Time.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Darian (Callisto)

Overview

The Galilean Darian calendars are calculated calendars created by Thomas Gangale for use on the four Galilean moons of Jupiter---Io, Europa, Ganymede, and Callisto. They are intended to loosely align with the Mars-based Darian calendar, roughly sharing an epoch as well as all 24 months.

The inner three moons are in a 2:4:8 Laplace resonance, and thus their orbits and solar days are in nearly exact ratios. As the solar day of Io, the inner-most moon, is over 42 hours, its day is broken into two units of time called 'circads' that are ~21 hours each and act as calendar days. The remaining moons have their orbits broken up into similar-sized circads: 4 for Europa, 8 for Ganymede, and 19 for Callisto.

The calendars all share circads, months, and weeks of 8 circads, though they drift in and out of phase with each other depending on intercalation. Generally months have 32 circads, with few exceptions, most notably the final month that can be intercalated to have 24, 32, or 40 circads. The months are also prefixed with a shorthand name of the moon.

Info

All four of the Galilean Darian calendars use roughly the same epoch, within a week, as the Martian Darian calendar, each of which corresponds with their Meridian Time.

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Darian (Titan)

Overview

The Darian calendar for Titan, moon of Saturn, was created by Thomas Gangale and is a continuation of the calendars created for Mars and the four Galilean moons of Jupiter. It is intended to closely align with the Martian Darian calendar, roughly sharing an epoch as well as all 24 months.

As the solar day of Titan is nearly 16 Earth days long, its day is broken into 16 units of time called 'circads' that are ~24 hours each and act as calendar days.

This calculated calendar features 24 months of 28 or 32 circads each, allowing for a clean division of the 8-circad week.

Leap years add an entire week, split evenly between the 12th and 24th months, adding 4 circads each for a total of 32 circads, allowing it to stay in sync with the Martian Darian calendar.

Years don't always begin and end at midnight on Titan's prime meridian, as the circad system takes precedence over the solar day.

Info

Accuracy

The accuracy of this calendar system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter in the Galilean calendars, but it isn't clear if it has also been accounted for in this calendar.

Source

This formula was extrapolated from the writings of Thomas Gangale found at this website.

It can be somewhat calibrated using this model if you know what you're doing.

Yuga Cycle (IST)

Overview

The Yuga Cycle is the cyclic age in Hindu cosmology. It is divided into 4 yugas, each divided into dawn, proper, and dusk periods, that lasts for a total of 4,320,000 years.

Each yuga in the cycle lasts for a shorter amount of time but, according to Hindu cosmology, also reduces the moral and phsical state of the world before a cataclysm and re-establishment of the dharma and restarting of the cycle.

The current yuga, Kali Yuga, is the shortest and worst. It will last until the year 428,899 CE.

Info

Accuracy

The Yuga Cycle is ultimately based off the Gregorian calendar via the Kali Ahargaṅa and is considered to be very accurate compared with historical records.

Source

All of the information for this timekeeping system has come from its Wikipedia article.

Sothic Cycle

Overview

The Sothic Cycle is the relationship between the start of the new year of the Egyptian calendar and the heliacal rising of the star of Sirius, which was originally what the calendar was based on.

The Egyptian calendar had years of exactly 365 days while the heliacal rising of Siruis is on a cycle of 365.25 days, causing the two to drift apart and eventually come together again over the course of 1460 years. This rate of error was one of the references for the creation of the Julian calendar, meaning the two calendars share the same relationship through the cycle. For every 1460 Julian years there are 1461 Egyptian years.

The first cycle is believed to begin on 27 June 2781 BCE, which is implied to be the date of the creation of the Egyptian calendar.

The Sothic Cycle was instrumental in calibrating the Egyptian date by historians.

Info

The Sothic Cycle shows a relationship between the Julian and Egyptian calendars of 1460/1461. The Gregorian calendar does not line up in the same way due to the revised leap year rules.

Accuracy

This calendar is mathematically exact, though my calculations are sometimes a day off from official sources. This could be due to leap day rules or due to astronomical dates.

Historians are also somewhat unsure about how many cycles have passed, and it's possible that there has been one more cycle before the accepted first cycle.

Source

All of the information for this timekeeping system has come from its Wikipedia article.

Olympiad

Overview

The Olympiad is an ancient unit of measurement that corresponds with 4 solar years, counting the number of Olympic games since the first Olympic games in 775 BCE. It was used for over 1000 years by Ancient Greece as well as Ancient Rome.

The Olympics eventually ceased to be held around 400 AD, and with it the Olympiad fell out of use.

Info

The ancient Olympiad has little connection to the modern olympic games, as they are out of phase with each other and start with a different epoch.

Accuracy

The exact start of the Olympic games is unclear, and most sources will just say it was held in the summer of 776 BCE. This website uses astronomical dates, so it is displayed here as 775 BCE. I have chosen to use 1 August of the Julian calendar for the middle of summer.

Source

All of the information for this timekeeping system has come from its Wikipedia article.

Pawukon (WITA)

Overview

The Pawukon calendar of Bali, Indonesia is a 210-day cycle of ten repeating and undulating "weeks" that happen concurrently. The first week has only 1 day, the second week has 2 days, the third week has 3 days, and so on.

Weeks 3, 5, 6, and 7 all simply cycle through their days. Weeks 4 and 8 also cycle but repeat the 71st day for days 72 and 73, a form of intercalation, in order to match the 210-day cycle. Week 9 also intercalates by repeating the 1st day for days 2, 3, and 4.

Weeks 1, 2, and 10 operate on more complex rules, involving adding the 'urip', a special number assigned to each day, of the days of Week 5 and Week 7 and adding 1. If the resulting number is even, then Week 1 and 2 are the first and second days respectively. If it's odd, then Week 2 is the first day and Week 1 is missing entirely. If this number matches the urip of any day in Week 10, then it is that day in Week 10.

Each of the 30 cycles of Week 7 also has a name. This calendar does not count the number of cycles and has no epoch.

Info

Accuracy

This calendar is a simple cycle of days with a known epoch and is expected to be exactly accurate.

Source

All of the information for this timekeeping system has come from its Wikipedia article.

Sexagenary Year (CST)

Overview

The Sexagenary Cycle is a system of counting years in the Chinese calendar (and several other aspects of life). It is a multiplication of the 10 Heavenly Stems and the 12 Earthly Branches (Chinese Zodiac) with half of the combinations left out, leading to a total cycle length of 60. The cycle moves to the next combination on the day of the New Year in the Chinese lunisolar calendar.

Info

Accuracy

This calendar system should be very accurate. It may be off by a few days at the start of a given year, or rarely an entire month, due to the inaccuracies from the Chinese lunisolar calendar calculations. However, it corrects itself by the next new moon.

Source

Some general information was taken from the Wikipedia article for this calendar, but the general calculation is derived from the Chinese lunisolar calendar.

Astronomical Data

Astronomical data includes calculations of celestial events and phenomena that are fundamental to many calendar systems. These include equinoxes, solstices, lunar phases, eclipses, and other astronomical measurements that serve as reference points for timekeeping systems.

Northward Equinox

Overview

This is the approximate date and time of this year's Northward Equinox. In the Northern Hemisphere this is known as the Spring Equinox. In the Southern Hemisphere it is known as the Fall Equinox. It is the time when the length of the day and night are equal all over the planet and the solar declination is heading northward.

Info

The Northward Equinox is an important starting point or anchor point in some calendars. It usually occurs around March 20th. Over time, roughly in a cycle of 25,772 years, Earth's axes precess, causing the equinoxes and solstices to slowly drift through the entire year.

Accuracy

The accuracy of this calculation depends on the precision of Meeus's calculations. On top of that, my solutions don't exactly match those provided by Meeus, either due to Javascript's base-2 calculations or due to misinterpreting steps such as adding Terrestrial Time. Overall these results are very close, usually within a few minutes of reality.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Northern Solstice

Overview

This is the approximate date and time of this year's Northern Solstice. In the Northern Hemisphere this is known as the Summer Solstice. In the Southern Hemisphere it is known as the Winter Solstice. It is the time when the Northern Hemisphere experiences its longest day while the Southern Hemisphere experiences its shortest day.

Info

The Northern Solstice usually occurs around June 20th. Over time, roughly in a cycle of 25,772 years, Earth's axes precess, causing the equinoxes and solstices to slowly drift through the entire year.

Accuracy

The accuracy of this calculation depends on the precision of Meeus's calculations. On top of that, my solutions don't exactly match those provided by Meeus, either due to Javascript's base-2 calculations or due to misinterpreting steps such as adding Terrestrial Time. Overall these results are very close, usually within a few minutes of reality.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Southward Equinox

Overview

This is the approximate date and time of this year's Southward Equinox. In the Northern Hemisphere this is known as the Fall Equinox. In the Southern Hemisphere it is known as the Spring Equinox. It is the time when the length of the day and night are equal all over the planet and the solar declination is heading southward.

Info

The Southward Equinox is an important starting point or anchor point in many calendars. It usually occurs around September 20th. Over time, roughly in a cycle of 25,772 years, Earth's axes precess, causing the equinoxes and solstices to slowly drift through the entire year.

Accuracy

The accuracy of this calculation depends on the precision of Meeus's calculations. On top of that, my solutions don't exactly match those provided by Meeus, either due to Javascript's base-2 calculations or due to misinterpreting steps such as adding Terrestrial Time. Overall these results are very close, usually within a few minutes of reality.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Southern Solstice

Overview

This is the approximate date and time of this year's Southern Solstice. In the Northern Hemisphere this is known as the Winter Solstice. In the Southern Hemisphere it is known as the Summer Solstice. It is the time when the Northern Hemisphere experiences its shortest day while the Southern Hemisphere experiences its longest day.

Info

The Southern Solstice is an important starting point or anchor point in some calendars. It usually occurs around December 20th. Over time, roughly in a cycle of 25,772 years, Earth's axes precess, causing the equinoxes and solstices to slowly drift through the entire year.

Accuracy

The accuracy of this calculation depends on the precision of Meeus's calculations. On top of that, my solutions don't exactly match those provided by Meeus, either due to Javascript's base-2 calculations or due to misinterpreting steps such as adding Terrestrial Time. Overall these results are very close, usually within a few minutes of reality.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Longitude of the Sun

Overview

This is the approximate longitude of the sun, the distance in degrees the Earth has traveled along its orbit since the last Northward Equinox.

Info

The longitude of the sun is an important factor in the Chinese lunisolar calendar and its derivatives. As a circle has 360 degrees and the year has roughly 365 days, the longitude of the sun increments a little less than 1 degree each day.

Accuracy

The accuracy of this calculation depends on the precision of Meeus's calculations, which should be very accurate.

Source

This calculation was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This Month's New Moon

Overview

This is the approximate time of the New Moon, also known as a Lunar Conjunction, of the current month.

A New Moon is when the dark side of the moon is directly facing the Earth, rendering it difficult to see. It takes place when the moon is closest to the sun relative to Earth.

It is an important event in many cultures, and most lunar or lunisolar calendars use the New Moon as the beginning of the month.

Info

Calculating the lunar phases is no easy task. It involves several steps and different tables of equations, and it is likely the most resource-taxing calculation on this site. Unfortunately it also must be calculated several times due to the nature of lunar calendars, though the date shown here is resused when possible.

New Moons are on average 29.53059 days apart, but that number can vary by several hours in a given cycle due to the shape of the moon's orbit as well as other gravitational effects. Thus, it is often necessary to calculate each New Moon directly.

Accuracy

This calculation is mostly accurate, but it differs from Jean Meeus's solutions by a few minutes. I am not sure why this is the case, though I suspect it has to do with the base-2 calculations in JavaScript. It is also possible that my Terrestrial Time calculations are independently incorrect, which are factored into the New Moon calculation. Dates thousands of years away from 2000 CE are likely to be significantly off.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

This Month's First Quarter Moon

Overview

This is the approximate time of the First Quarter Moon of the current month.

A First Quarter Moon is when the Eastern half of the moon is in sunlight from the perspective of Earth.

Info

Calculating the lunar phases is no easy task. It involves several steps and different tables of equations, and it is likely the most resource-taxing calculation on this site.

First Quarter Moons are on average 29.53059 days apart, but that number can vary by several hours in a given cycle due to the shape of the moon's orbit as well as other gravitational effects.

Accuracy

This calculation is mostly accurate, but it differs from Jean Meeus's solutions by a few minutes. I am not sure why this is the case, though I suspect it has to do with the base-2 calculations in JavaScript. It is also possible that my Terrestrial Time calculations are independently incorrect, which are factored into the New Moon calculation. Dates thousands of years away from 2000 CE are likely to be significantly off.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

This Month's Full Moon

Overview

This is the approximate time of the Full Moon, also known as a Lunar Opposition, of the current month.

A Full Moon is when the light side of the moon is directly facing Earth, making it the brightest object in the night sky. It takes place when the moon is directly opposite the sun relative to Earth.

Info

Calculating the lunar phases is no easy task. It involves several steps and different tables of equations, and it is likely the most resource-taxing calculation on this site.

Full Moons are on average 29.53059 days apart, but that number can vary by several hours in a given cycle due to the shape of the moon's orbit as well as other gravitational effects.

Accuracy

This calculation is mostly accurate, but it differs from Jean Meeus's solutions by a few minutes. I am not sure why this is the case, though I suspect it has to do with the base-2 calculations in JavaScript. It is also possible that my Terrestrial Time calculations are independently incorrect, which are factored into the New Moon calculation. Dates thousands of years away from 2000 CE are likely to be significantly off.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

This Month's Last Quarter Moon

Overview

This is the approximate time of the Last Quarter Moon of the current month.

A Last Quarter Moon is when the Western half of the moon is in sunlight from the perspective of Earth.

Info

Calculating the lunar phases is no easy task. It involves several steps and different tables of equations, and it is likely the most resource-taxing calculation on this site.

Last Quarter Moons are on average 29.53059 days apart, but that number can vary by several hours in a given cycle due to the shape of the moon's orbit as well as other gravitational effects.

Accuracy

This calculation is mostly accurate, but it differs from Jean Meeus's solutions by a few minutes. I am not sure why this is the case, though I suspect it has to do with the base-2 calculations in JavaScript. It is also possible that my Terrestrial Time calculations are independently incorrect, which are factored into the New Moon calculation. Dates thousands of years away from 2000 CE are likely to be significantly off.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Next Solar Eclipse

Overview

A solar eclipse occurs when the moon casts a shadow anywhere onto the Earth. It is a fairly rare event that always occurs during a New Moon.

When the moon completely covers the disk of the sun from the perpective of a point on the Earth, it is called a Total Solar Eclipse. When the moon appears to be smaller than the sun, with the sun visible in a 'Ring of Fire' around the edge, it is called an Annular Solar Eclipse. When the moon covers part of the sun but doesn't intersect the center of the sun's disk, it is called a Partial Solar Eclipse.

This entry also displays the node at which the eclipse took place as well as the hemisphere of Earth where it is visible at its maximum.

Solar eclipses typically are only viewable from a small area on the Earth's surface, and they are historically significant events that have inspired legend, religion, and myth. The solar eclipse of 8 April 2024 CE is what inspired me to build this website.

Info

Solar eclipses happen two to four times per year, though total solar eclipses are rarer (every two or three years). They can only occur when a New Moon happens very near to the moon's ascending or descending nodes—the points along the lunar orbit that intersect the Earth's equator. These are at 0°/360° and 180° respectively.

Due to the oblong shape of the moon's orbit, the moon can either appear larger or smaller than the sun from the perspective of Earth. This coincidence produces the two different types of central eclipses, total and annular.

During a total solar eclipse, the sun's corona is visible to the naked eye, providing a spectacular sight as well as an opportunity to conduct science.

Accuracy

This calculation is reasonably accurate (to within seconds or minutes) for thousands of years before and after the year 2000 CE. Outside of that, errors are induced which can grow from hours to even days.

This calculation relies on the New Moon calculation as well as Terrestrial Time, each of which have potential to induce these errors.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Next Lunar Eclipse

Overview

A lunar eclipse occurs when the Earth casts a shadow anywhere onto the moon. It is a fairly rare event that always occurs during a Full Moon.

When the moon is completely covered by Earth's shadow (also known as the Umbra), it is called a Total Lunar Eclipse. When the moon only partially intersects the Umbra, it is called a Partial Lunar Eclipse. When the Earth only covers a part of the sun's disk from the perspective of the moon, it is called a Penumbral Lunar Eclipse.

This entry also displays the node at which the eclipse took place as well as the hemisphere of Earth that it is above at its maximum.

Lunar eclipses typically are viewable from a large area on the Earth's surface, and they are historically significant events that have inspired legend, religion, and myth.

Info

Lunar eclipses happen two to four times per year. Penumbral eclipses typically aren't noticeable from Earth, as the lunar surface is still quite bright. The moon must enter deep within the Penumbra or into the Umbra before it is able to be noticed with the naked eye. While in the Umbra, the moon's surface often takes on a reddish color due to light scattering through the Earth's atmosphere (similar to a sunset), being described as a 'blood moon' in historical literature.

Accuracy

This calculation is reasonably accurate (to within seconds or minutes) for thousands of years before and after the year 2000 CE. Outside of that, errors are induced which can grow from hours to even days.

This calculation relies on the New Moon calculation as well as Terrestrial Time, each of which have potential to induce these errors.

Source

This calculation in its entirety was sourced from Astronomical Algorithms (1991) by Jean Meeus.

This cycle can be calibrated using the ephemerides at this website.

Lunisolar Calendars

Lunisolar calendars combine both lunar and solar elements to track time. They use lunar months (based on moon phases) but add intercalary months to keep the calendar synchronized with the solar year. This ensures that seasonal events occur at roughly the same time each year.

Chinese (CST)

Overview

The Chinese calendar is an observational lunisolar calendar used by much of East Asia with a long history dating back to ancient China.

It features numerically-named months (月 "yue") of 29 or 30 days (日 "ri") that begin on the same day as the New Moon in China (CST), with an intercalary month added on leap years that happen roughly every 2 or 3 solar years (年 "nian"). Years are also named in a 12-year cycle of the 12 Earthly Branches (Chinese Zodiac).

Different versions of this calendar use different eras, but this website uses 2697 BCE as the Year of the Yellow Emperor (2698 BC), a date which was standardized by Sun Yat-sen in 1912 despite there being controversy over the exact date.

Info

The Chinese calendar is one of the most widespread calendars in history, used by various cultures in Asia and around the world either directly or as a base.

Accuracy

Calculating this calendar is very difficult and requires calculating the Winter Solstice, Longitude of the Sun, and any given New Moon. Due to the difficulty of this calculation, months and days might be off by 1 at times, though they typically self-correct by the next month. Rarely, the leap month may be incorrect, especially in cases where the new moon and solstice happen on the same day.

Source

This equation was based off of the steps found here.

General information was taken from the Wikipedia article for this calendar.

Đại lịch (ICT)

Overview

The Đại lịch calendar is a traditional observational lunisolar calendar of Vietnam. It is derived from the Chinese lunisolar calendar and shares many of the same elements, but it is set to Vietnamese time, meaning on rare occasions the two calendars can temporarily be significantly offset, only to realign again later.

It features 12 months of 29 or 30 days with a leap month on average every 2-3 years.

The Đại lịch calendar also follows a similar 12 Earthly Branches (Vietnamese Zodiac) theme for each year, though a few of the animals are different from the Chinese calendar.

This calendar uses the same epoch as the Gregorian calendar and may not reflect historic epochs. Similarly, the calendar hasn't always been set to Vietnamese time, changing back from Chinese time in the mid-20th century, so dates before that are likely to be incorrect.

Info

Accuracy

This calendar relies on the same calculations as the Chinese Lunisolar Calendar, though using midnight in Vietnam as the start of the say. Thus, it may vary mildly or significantly from the Chinese calendar. This may or may not itself be accurate, but the result should be relatively close to reality.

Source

Some general information was taken from the Wikipedia article for this calendar, but the general calculation is derived from the Chinese lunisolar calendar.

Dangun (KST)

Overview

The Dangun calendar is a traditional observational lunisolar calendar of Korea. It is no longer officially used, but it is still maintained by the South Korean goverment for cultural purposes and holidays. It is derived from the Chinese lunisolar calendar where it gets its months (월) and days (일) while years (년) are counted from 2333 BCE.

The Dangun calendar is calculated based on midnight in Korea, and its dates may misalign, sometimes significantly, from the Chinese lunisolar calendar.

Info

The Dangun calendar epoch was not traditionally used, but it has seen unofficial use in the 20th century.

Accuracy

This calendar relies on the same calculations as the Chinese Lunisolar Calendar, though using midnight in Korea as the start of the say. Thus, it may vary mildly or significantly from the Chinese calendar. This may or may not itself be accurate, but the result should be relatively close to reality.

Source

Some general information was taken from the Wikipedia article for this calendar, but the general calculation is derived from the Chinese lunisolar calendar.

Hebrew (IST)

Overview

The Hebrew calendar is a calculated lunisolar calendar used by the Jewish faith for religious and celebratory purposes, and it is also an official calendar of Israel.

It features 12 months of 29 or 30 days that start approximately on the day of the New Moon, referred to as a Molad. It has an intercalary 13th month added after the month of Adar, called Adar II, based on the Metonic cycle which places 7 leap years in every cycle of 19 years.

Years are denoted with AM for 'Anno Mundi', meaning 'in the year of the world', referring to the Jewish date of Creation around the year 3760 BCE.

Info

The Hebrew calendar is not strictly based on the moon. The modern Jewish calendar, which is a mathematical equation, was codified by Rabbi Hillel II in the year 358 CE. Days start at sunset, which is approximated as 18:00 in Israel.

Accuracy

Though it is a complex equation, the calculation for this calendar is exactly accurate to modern Jewish calendars. However, prior to the Jewish year 4119 (358 CE), the calendar was decided through observation.

Source

The main calculation of this calendar came from this website.

Dates can be checked for calibration here.

Some general information was taken from the Wikipedia article for this calendar.

Babylonian (AST)

Overview

The Babylonian calendar is an observational lunisolar calendar that was used in ancient Babylon. It consists of 12 months of 29 or 30 days that begin after the first sighting of the new crescent moon at sunset. An intercalary month is added every 2-3 years in accordance with the 19-year Metonic cycle, typically repeating the 12th month but repeating the 6th month for only the 17th year of the Metonic cycle.

Days start at sunset. While there is no week structure, "holy-days" take place at intervals of 7 days after the start of each month to make offerings to specific gods; Marduk and Ishtar on the 7th, Ninlil and Nergal on the 14th, Sin and Shamash on the 21st, and Enki and Mah on the 28th.

Due to the long history of empires and conquests of Babylon, years can be counted from a number of different epochs. Two are displayed here: the Seleucid Era (SE) denoting the time since the founding of the Seleucid Empire, and the Asarcid Era (AE) denoting the time from the start of the Arsacid Empire.

Info

The Babylonian calendar is derived from the ancient Sumerian calendar and shares many similarities in month names, while the Hebrew calendar is derived from the Babylonian calendar.

Accuracy

The Babylonian calendar is well-documented, but there is some debate as to how it should be calibrated. Certain dates are well-defined, but since it is largely an observational calendar, then there is no set formula to determine the start of each month. This site uses 6pm in Babylon as the time of sunset, and the start of a month is calculated as being the first sunset that occurs at least 24 hours after a new moon.

The dates that result from this calculation line up very closely with date tables found from various sources, but not exactly. They also vary amongst each other. It is unclear how these tables were calculated, so it's possible that any or all of these calculations are wrong in some way.

The Metonic cycle wasn't established in the Babylonian calendar until around 380 BCE. Leap years prior to that date aren't handled with historical accuracy. Similarly, dates after 75 CE are calculated but not historical.

The intercalary month of 𒌚𒋛𒀀𒆥 was not explicitely named but directly described. The name was chosen to match the pattern found in the other intercalary month, 𒌚𒋛𒀀𒊺.

Source

Much of the information on this calendar came from its Wikipedia article.

The calendar can be calibrated using this converter which is sourced from this table of dates.

Epirote (EET)

Overview

The Epirote calendar is the previously-undescribed Greek lunisolar calendar displayed on the Antikythera mechanism, theorized to be from the Epirus region of Ancient Greece.

It is a calculated lunisolar calendar with 12 months of 29 or 30 days, with an intercalary month added every 2-3 years in accordance with a variant of the Metonic cycle (year 1 instead of year 19). The exact intercalary month is unknown, but it is theorized to be the 4th month of ΜΑΧΑΝΕΥΣ.

Months were either "full" (30 days) or "hollow" (29 days) in a roughly alternating pattern; however, hollow months still had days that numbered 1-30, but one day was skipped entirely, starting with the 1st day of the 1st month of the Metonic cycle and then skipping a day every 64 days on a cycle of 47 months (repeating 5 times per Metonic cycle). Thus, the first day of each Metonic cycle is skipped and begins on the 2nd day of ΦΟΙΝΙΚΑΙΟΣ.

There is no known epoch for the Epirote calendar, but this site uses the calibration date of the Antikythera mechanism, 20 August 204 BCE, as a pseudoepoch, listed here in parentheses.

Info

When the Antikythera mechanism was discovered in the early 1900s, it shocked historians as proof that the Ancient Greeks had the technology to create precision bronze gears. While large portions of the mechanism were damaged or missing, enough remained that enabled 21st century researchers, with careful deduction and investigation, to rebuild and sufficiently calibrate its calendar.

Accuracy

The Epirote calendar is an enigma in horology. Many things are known about it thanks to research done on the Antikythera mechanism, but ultimately certain aspects of the calendar, such as its calibration date and leap month, are educated speculation.

Otherwise the implementation of the algorithm is fairly straightforward, if a bit complex. As this calendar was calibrated millennia ago, it is sure to have experienced significant drift from the solar year, which is unaccounted for here.

Source

Research into this calendar spans several scientific papers and studies.

Information about its month cycle can be found here.

Here is a paper that used deductive reasoning to locate the source of the calendar in Epirus.

This paper shows a lot of research about how the calibration date was uncovered.

And finally, Clickspring has a fantastic YouTube series about building a replica of the Antikythera mechanism using tools from the time period. He is also a leading researcher referenced in some of the other papers.

Lunar Calendars

Lunar calendars are based primarily on the phases of the moon, with months corresponding to lunar cycles. Unlike lunisolar calendars, they do not attempt to synchronize with the solar year, so dates drift through the seasons over time.

Umm al-Qura (AST)

Overview

The Umm al-Qura calendar is an observational lunar calendar used in Saudi Arabia. It is the predictive version of the Islamic Hijri calendar, perhaps the only extant true lunar calendar in the world. It features 12 lunar months of 29 or 30 days, with days starting at sunset, for a total of 355 or 356 days per year, causing it to be out of sync with solar calendars.

Era dates are denoted 'AH' from 'Anno Hegirae', meaning 'In the year of the Hijrah'. Each month starts shortly after the New Moon when it begins to appear as a crescent.

The desert-faring culture of Islam is apparent in this calendar, as such a civilization is less affected by seasonal changes than civilizations in most other biomes. Thus, they would have had no need to implement an intercalary month system to synchronize the calendar with the solar year.

Info

The Hijri calendar is on a roughly 37-year cycle when compared with the solar year. Dates and holidays drift throughout the entire year before arriving back where they started 37 years prior.

Accuracy

Many Muslim nations have their own rules for determining the start of the month, often based on direct observation, and as a result their calendar dates may occasionally misalign. The algorithm used by this website requires calculating the New Moon and uses 18:00 local time in Mecca for sunset. Its accuracy is dependent on the New Moon calculations and may not reflect historical records.

Source

A lot of the information about this calendar came from its Wikipedia article.

This site seems to be a good source for callibrating dates.

Solilunar Calendars

Solilunar calendars are a unique type that combines solar and lunar elements as their base units. However, unlike lunisolar calendars, solilunar calendars use the solar year as the main unit, with months being determined secondary based on the moon's sidereal month.

Togys Esebi (KZT)

Proposed Calendars

Proposed calendars are calendar reform systems that have been suggested as alternatives to the Gregorian calendar. These proposals aim to address various issues with the current calendar system, such as irregular month lengths, shifting day-of-week patterns, or religious influences. While none have been officially adopted, they represent interesting approaches to calendar design.

Human Era

Overview

The Human Era, also known as the Holocene Era, is a Gregorian-based calendar proposed by Cesare Emiliani in 1993 CE. It is the representation of time since the beginning of the Holocene and the Neolithic Revolution, when humans started living in fixed agricultural settlements.

Info

The Human Era is an attempt to adapt the Gregorian calendar, which has become widespread enough that it could be considered the default calendar of the world, to an epoch that better encompasses human history. It chooses its epoch based on what may be the most significant moment in our ancient past: the time when humans first created civilization. Conventiently, this occurs roughly 10000 years before 1 AD, allowing for simple math to arrive at the converted date.

Accuracy

As this calendar is only a proposal, there really isn't anything to compare it to historically. It is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Invariable

Overview

The Invariable calendar is a Gregorian-derived calendar proposed by L. A. Grosclaude in 1900 CE as well as by Gaston Armelin in 1887 CE. It features months in a repeating pattern of 30/30/31 days with New Years Day happening between December and January and Leap Day occurring between June and July in leap years, which happen in the same years as the Gregorian calendar. These two special days are not part of any week nor month, as if the calendar has paused for 24 hours.

The regular month lengths ensure that the first of every month always lands on a Monday, Wednesday, or Friday in a predictable pattern that is the same every year.

Info

Accuracy

As this calendar is only a proposal, there really isn't anything to compare it to historically. It is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

The World Calendar

Overview

The World Calendar was a Gregorian-derived calendar proposed by Elisabeth Achelis in 1930 CE and was nearly adopted by the League of Nations. It features months in a repeating pattern of 31/30/30 days with World's Day happening between December and January and Leapyear Day occurring between June and July in leap years, which happen in the same years as the Gregorian calendar. These two special days are not part of any week nor month, as if the calendar has paused for 24 hours.

The regular month lengths ensure that the first of every month always lands on a Sunday, Wednesday, or Friday in a predictable pattern that is the same every year.

Info

Accuracy

As this calendar is only a proposal, there really isn't anything to compare it to historically. It is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Symmetry454

Overview

The Symmetry454 calendar is a Gregorian-derived calendar proposed in 2005 by Irv Bromberg.

It features the same months as the Gregorian calendar but of different lengths, following a pattern of 28/35/28 days. This allows for a whole number of weeks in each month, 4/5/4 respectively, hence the calendar's name.

The calendar features a leap year that extends December by an extra week, occurring every 5 or 6 years. This keeps it in line with the Gregorian calendar on a 293-year cycle containing 52 leap years.

The format of the calendar allows each day of the year to always occur on the same day of the week.

Info

This calendar is calibrated using 1 January 2001 as a reference date.

Accuracy

As this calendar is only a proposal, there really isn't anything to compare it to historically. It is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

It has also been calibrated using the Kalendis tool which was created by the same creator of Symmetry454.

There are sources that list this calendar as meeting up with the Gregorian calendar on 1 January 2005 CE, but I believe this to be a typo. The dates line up on 1 January 2001 CE as well as 1 January 1 CE, which is the epoch in both calendars.

Source

Much of the information on this calendar came from its Wikipedia article.

This calendar has also been calibrated using a Windows app created by the creator of the Symmetry454 calendar, which can be found here.

Symmetry010

Overview

The Symmetry010 calendar is a Gregorian-derived calendar proposed by Irv Bromberg.

It features the same months as the Gregorian calendar but of different lengths, following a pattern of 30/31/30 days. This allows for each quarter of every year to follow the same pattern of 13 weeks or 91 days.

The calendar features a leap week that comes after December (humorously labeled as "Irv" by its creator), occurring every 5 or 6 years. This keeps it in line with the Gregorian calendar on a 293-year cycle containing 52 leap years.

The format of the calendar allows each day of the year to always occur on the same day of the week.

Info

This calendar is calibrated using 1 January 1 CE as a reference date.

Accuracy

Being a simple calculation of days with a known epoch, this calendar is expected to be exactly accurate.

Source

The information for this calendar came from this website.

Positivist

Overview

The Positivist calendar was a Gregorian-derived calendar proposed by French philosopher and Positivist Auguste Comte in 1849. It features 13 months of 28 days named after significant figures in Western history.

Each of the 364 days is also named after a historical figure, not all of whom are intended to be remembered as heroes but also as villains. Days of the week are still carried over from the Gregorian calendar, though the final day of the year, The Festival of All the Dead, is not part of a day of the week, as is the following day during leap years, The Festival of Holy Women. Leap year rules follow the Gregorian calendar.

Years are counted from 1789, the year of the French Revolution, and are denoted as "Year of the Great Crisis". Each year starts on January 1 of the Gregorian calendar.

Info

The Positivist calendar includes 364 days named after historical figures, organized into 13 months. Each day is named after a significant person from Western history, including philosophers, scientists, artists, and political figures. The calendar was designed to celebrate human achievement and progress.

Accuracy

As this calendar is only a proposal, there really isn't anything to compare it to historically. It is intrinsically based on and locked to the Gregorian calendar, making it perfectly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Decimal Time

Decimal time systems use base-10 divisions instead of the traditional base-60 (hours/minutes/seconds) or base-12/24 (hours) systems. These systems aim to simplify time calculations by using decimal arithmetic throughout.

French Revolutionary

Overview

Revolutionary Time is the timekeeping system employed by France during the French Revolution from 1794 to 1800. It divides the day into 10 hours, each hour into 100 minutes, and each minute into 100 seconds.

The French would have used Paris Mean Time (GMT + 00:09:21) but this website uses local time.

Info

Accuracy

As this is a simple mathematical calculation, this clock is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

.beat (BMT)

Overview

.beat time, also known as Swatch Internet Time, is a timekeeping system developed in 1998 by the Swatch corporation. It divides the day into 1000 equal parts, called .beats, and is set to the BMT timezone (UTC +1).

Info

Accuracy

As this is a simple mathematical calculation, this clock is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Hexadecimal

Overview

Hexadecimal time is a simple representation of the current fraction of a day in hexadecimal. Midnight starts at .0000 and the moment just before midnight is .FFFF. The smallest unit of resolution is 675/512 seconds, or about 1.318 seconds.

Info

Accuracy

As this is a simple mathematical calculation, this clock is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Binary (16 bit)

Overview

Binary time is the binary representation of the day divided into 2^16 (65,536) equal parts, with all 0s being midnight and a 1 followed by 15 zeros being exactly half the day (noon). The smallest unit of resolution is 675/512 seconds, or about 1.318 seconds.

Info

Accuracy

As this is a simple mathematical calculation, this clock is exactly accurate.

Source

All of the information on this calendar came from its Wikipedia article.

Other Time

Other time systems include various alternative approaches to timekeeping, such as time systems for other planets, moons, or unique measurement methods that don't fit into traditional categories.

Coordinated Mars Time

Overview

Coordinated Mars Time, also called MTC as well as Airy Mean Time (AMT), is a proposed clock for use on Mars which has gained some level of mainstream traction in the scientific community. It is intended to be a Martian analog to Earth's UTC.

The time is displayed as hours, minutes, and seconds since midnight on Mars at the location of the Airy-0 crater. The clock is the same as clocks on Earth, with 24 hours and 60 minutes in an hour, though each unit is slightly longer due to the length of the sol being 39 minutes and 35 seconds longer than the day.

Info

This clock uses the Mars Sol Date for the calculation determining where midnight begins.

Accuracy

This clock should be reasonably accurate, though it might be off by a few minutes or seconds due to Terrestrial Time.

Source

All of the information on this calendar came from its Wikipedia article.

Io Meridian Time

Overview

Io Meridian Time is a measure of time passed since midnight on the prime meridian of Io, moon of Jupiter.

It features a similar 24-hour clock to Earth time, but the units are about 11.5% shorter. One Io solar day is about two Earth days, so the day is further broken up into two circads of 21 hours each.

Io is tidally locked with Jupiter, meaning one side of the moon always faces the planet and the other side always faces away. The prime meridian is determined to be the meridian on the moon's surface that is facing directly at Jupiter.

Midnight is thus the time when the moon is directly between Jupiter and the sun, though this is only used as an epoch for the beginning of the first circad in each solar day. The second circad happens when the moon is on the opposite side of Jupiter from the sun.

Io Meridian Time is a name that was chosen for this website and might not be accurate.

Info

Io is in a 2/4/8 Laplace resonance with Europa and Ganymede, so their solar days are equally comprised of 2/4/8 circads, though the length of their circads are very slightly different. The circad of Io is 21.23833 Earth hours long, which is then broken into 24 Io hours.

As the orbit of Io is not very inclined, midnight on Circad 1 is also the time of a total solar eclipse on Jupiter.

Accuracy

The accuracy of this timekeeping system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

The name of this timekeeping system is my own creation, as Mr. Gangale did not give it a name himself.

Source

All of the information on this calendar came from its Wikipedia article.

Europa Meridian Time

Overview

Europa Meridian Time is a measure of time passed since midnight on the prime meridian of Europa, moon of Jupiter.

It features a similar 24-hour clock to Earth time, but the units are about 11.5% shorter. One Europa solar day is about four Earth days, so the day is further broken up into four circads of 21 hours each.

Europa is tidally locked with Jupiter, meaning one side of the moon always faces the planet and the other side always faces away. The prime meridian is determined to be the meridian on the moon's surface that is facing directly at Jupiter.

Midnight is thus the time when the moon is directly between Jupiter and the sun, though this is only used as an epoch for the beginning of the first circad in each solar day. The third circad happens when the moon is on the opposite side of Jupiter from the sun.

Europa Meridian Time is a name that was chosen for this website and might not be accurate.

Info

Europa is in a 2/4/8 Laplace resonance with Io and Ganymede, so their solar days are equally comprised of 2/4/8 circads, though the length of their circads are very slightly different. The circad of Europa is 21.32456 Earth hours long, which is then broken into 24 Europa hours.

As the orbit of Europa is not very inclined, midnight on Circad 1 is also roughly the time of a total solar eclipse on Jupiter.

Accuracy

The accuracy of this timekeeping system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

The name of this timekeeping system is my own creation, as Mr. Gangale did not give it a name himself.

Source

All of the information on this calendar came from its Wikipedia article.

Ganymede Meridian Time

Overview

Ganymede Meridian Time is a measure of time passed since midnight on the prime meridian of Ganymede, moon of Jupiter.

It features a similar 24-hour clock to Earth time, but the units are about 11.5% shorter. One Ganymede solar day is about eight Earth days, so the day is further broken up into eight circads of 21 hours each.

Ganymede is tidally locked with Jupiter, meaning one side of the moon always faces the planet and the other side always faces away. The prime meridian is determined to be the meridian on the moon's surface that is facing directly at Jupiter.

Midnight is thus the time when the moon is directly between Jupiter and the sun, though this is only used as an epoch for the beginning of the first circad in each solar day. The fifth circad happens when the moon is on the opposite side of Jupiter from the sun.

Ganymede Meridian Time is a name that was chosen for this website and might not be accurate.

Info

Ganymede is in a 2/4/8 Laplace resonance with Io and Europa, so their solar days are equally comprised of 2/4/8 circads, though the length of their circads are very slightly different. The circad of Ganymede is 21.49916 Earth hours long, which is then broken into 24 Ganymede hours.

As the orbit of Ganymede is not very inclined, midnight on Circad 1 is also roughly the time of a total solar eclipse on Jupiter.

Accuracy

The accuracy of this timekeeping system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

The name of this timekeeping system is my own creation, as Mr. Gangale did not give it a name himself.

Source

All of the information on this calendar came from its Wikipedia article.

Callisto Meridian Time

Overview

Callisto Meridian Time is a measure of time passed since midnight on the prime meridian of Callisto, moon of Jupiter.

It features a similar 24-hour clock to Earth time, but the units are about 11.5% shorter. One Callisto solar day is about nineteen Earth days, so the day is further broken up into nineteen circads of 21 hours each.

Callisto is tidally locked with Jupiter, meaning one side of the moon always faces the planet and the other side always faces away. The prime meridian is determined to be the meridian on the moon's surface that is facing directly at Jupiter.

Midnight is thus the time when the moon is directly between Jupiter and the sun, though this is only used as an epoch for the beginning of the first circad in each solar day.

Callisto Meridian Time is a name that was chosen for this website and might not be accurate.

Info

Callisto is in a 2/4/8 Laplace resonance with Io, Europa, and Ganymede, so their solar days are equally comprised of 2/4/8 circads, though the length of their circads are very slightly different. The circad of Callisto is 21.16238 Earth hours long, which is then broken into 24 Callisto hours.

As the orbit of Callisto is not very inclined, midnight on Circad 1 is also roughly the time of a total solar eclipse on Jupiter.

Accuracy

The accuracy of this timekeeping system is wholely dependent on the writings and calculations of Thomas Gangale. It is likely that these calculations weren't precise enough to extend more than a few decades, as they do seem to drift from ephemeris data.

The epoch is noted to account for the time it takes light to travel from Jupiter.

The name of this timekeeping system is my own creation, as Mr. Gangale did not give it a name himself.

Source

All of the information on this calendar came from its Wikipedia article.

Pop Culture

Pop culture time systems are fictional or entertainment-based timekeeping methods from movies, games, and other media. These systems often have unique rules and mechanics that reflect their fictional universes.

Minecraft Time

Overview

One day/night cycle in Minecraft is exactly 20 minutes. Days typically start when the player wakes up from their bed, and although there is a clock in the game, it has little information to expand upon that. This representation of Minecraft time divides the Minecraft day into 24-hour segments with minutes and seconds, set to midnight in the real world when it also resets the day counter.

Info

Time in Minecraft, and many games for that matter, is counted in ticks, which are the game loop cycles. One tick is 50ms, allowing for a rate of 20Hz. These are then counted and converted into game time.

Accuracy

This clock should be perfectly accurate, with the caveat that time in an actual Minecraft game can vary due to the fact that players can skip the night by sleeping in a bed.

Source

This calculation was sourced from the Minecraft Fandom Wiki.

Dream Time

Overview

According to the movie Inception, time in a dream is experienced 20 times slower, allowing for several days to be experienced in a single night's sleep. The time displayed here is the current time in your dream if you had begun sleeping at midnight.

Info

The concept of time dilation in dreams is actually a fascinating area of study, with some results showing little or no dilation while others show differences in reaction times in dreams. Whatever the real dilation ratio may be, it is nowhere near that expressed in the film.

Accuracy

This timekeeping system should be perfectly accurate but the epoch will not be the same for everyone, as people don't all sleep at the same time.

Source

This calculation was sourced from the movie Inception.

Termina Time

Overview

Termina Time is the timekeeping system found in The Legend of Zelda: Majora's Mask.

It features a cycle of 3 days of 24 hours that are each 150 seconds long when slowed. Days start at sunrise at 6:00am, so 5:00 of one day is followed by 6:00 of the next day.

Info

Slowed time was chosen over regular time due to the former fitting cleanly within three real life hours. The entire 3-day cycle in regular time takes 54 minutes, which would create an awkward short cycle at each real life day change (or rather, 6:00am).

Accuracy

This timekeeping system should be perfectly accurate based on local time.

Source

This calculation was sourced from The Legend of Zelda: Majora's Mask.

Stardate

Overview

Stardate is the timekeeping system found in Star Trek. Being a fictional system without an exact algorithm, it isn't even clear what Stardates are measuring. They also vary from series to series.

This calculation is based on the Stardate 0 epoch of 25 April 2265 from the Original Series, with a peculiar 7.21 stardates per day.

Info

Stardates were originally intended to be a system similar to the revised Julian Day Number, but the writers of the show weren't required to follow an exact algorithm.

Accuracy

Since this calendar is both fictional and without an exact algorithm, it is impossible to know how accurate this calculation is.

Source

This calculation was sourced from Star Trek.

Tamrielic

Overview

The Tamrielic calendar is a fictional Gregorian-derived calendar used in the Elder Scrolls franchise. It is a simple 1-1 mapping to the Gregorian calendar, including its leap years.

It has 12 months of 28-31 days along with weeks of seven days. Since the timeline is fictional, it is impossible to map it to a specific epoch. This site assumes the Tamrielic calendar shares a New Year with the Gregorian.

Info

Accuracy

Since this calendar is a simple mapping to the Gregorian calendar, it is perfectly accurate.

Source

This calculation was sourced from The Elder Scrolls.

Imperial Dating System

Overview

The Imperial Dating System is the date formatting system used in the Warhammer 40k universe. It is intrinsically based on the Gregorian calendar for its epoch and years.

The first digit represents the in-universe location of the event, with 0 representing events that took place on Terra (Earth). The next set of 3 digits represent the fraction of the current year that has passed, in hundredths.

After that, the final set of 3 digits represents the current year in the Gregorian calendar, truncated to the last three digits. The current millennium, denoted with a capital M, is the current millennium in the Gregorian calendar, starting with M1 at the year 0 BCE.

Info

Since this website is located on Terra (Earth), the first digit will always be 0.

This calendar considers the beginning of a millennium to be on year 1 (e.g. 2001 rather than 2000), so the final year of any millennium is written as 000.

Accuracy

This calculation is exactly accurate, as it is based on the Gregorian calendar and its year calculations. There is a bit of uncertainty regarding the handling of years before the Gregorian epoch, but that is outside the range of this calendar anyway.

Source

This calculation was sourced from the Warhammer 40k Fandom Wiki.

Shire (UTC)

Overview

The Shire calendar is a fictional calculated solar calendar used in the Lord of the Rings franchise. It is one of the calendars of the Hobbits. It is a solar calendar with 12 months of 30 days plus 5 or 6 named days.

Years begin in the winter with the special day of 2 Yule, taking place immediately after 1 Yule, which is the last day of the previous year. In the middle of the year between the months of Forelithe and Afterlithe are the special days of 1 Lithe, Mid-year's Day, and 2 Lithe. On leap years, the day of Overlithe is added after Mid-year's Day.

Leap years happen every 4 years but not on centennial years. Years are denoted with S.R. referring to Shire Reckoning, the count of years from the migration of Hobbits to the Shire from Bree in December 523 BCE.

Info

The Shire calendar took inspiration from many European calendars, including the Julian and Gregorian calendars for leap year rules as well as German calendars for month names and year structure. The leap year rules make its drift from the equinoxes a bit less than the Julian calendar but a bit more than the Gregorian.

Accuracy

Many would be surprised to learn that Tolkein intended for the Lord of the Rings and its companion novels to be historical texts, taking place in the same universe as the real world and translated by Tolkein from their original language into English.

However, despite making several highly-detailed calendar systems for the races and nations in his stories, Tolkein was not specific as to when any of the dates were supposed to occur. In various appendices, letters, and interviews, he gave figures for the War of the Ring, corresponding to S.R. 1419, happening anywhere from around 4000 BCE to 6000 BCE.

Without an anchor date, calculating this calendar is impossible, and every attempt to do so must make some sort of trade-off. There are fan sites that place Mid-year's Day on the day of the summer solstice each year, but that requires breaking the leap year rules of the calendar. Other sites place 2 Yule of S.R. 1419 at 1 January 4000 BC in either the Gregorian or Julian calendar. But since the leap year rules match neither the Gregorian nor Julian calendars, then this date is entirely arbitrary.

While Tolkein's use of dates of events in the books is surprisingly consistent, calendar algorithms are not the only way to keep track of time; Tolkein also kept meticulous notes of lunar phases and made several references to them throughout the story. These references were enough for Brandon Rhodes to uncover the fact that Tolkein used a 1942 CE lunar almanac to align his phases, specifically placing 2 Yule S.R. 1419 on 25 December 1941 CE.

Using the moon calculations on this site, I was unable to find a pattern of moons that closely matched those of 1942 CE within 2000 years +/- 4000 BCE, meaning Tolkein's estimate of when his story took place could not possibly be consistent with what he wrote in his story.

Thus, my conclusion is that 2 Yule S.R. 1 took place on 15 December 523 CE. Calculating the remainder of the calendar is simply a matter of following the algorithm clearly established in Tolkein's writings.

Source

As stated in the Accuracy tab, much of the work that went into calibrating this calendar was thanks to Brandon Rhodes.

Month and day names as well as general structure can be found here.

Political Cycles

Political cycles are timekeeping systems based on political events, terms of office, and governmental periods. These systems track the passage of time through political milestones rather than astronomical or calendar-based measurements.

US Presidential Terms

Overview

The term of the US president lasts 4 years, starting from January 20th at noon and ending January 20th at noon four years later. This is a running count of how many presidential terms have passed since the inauguration of George Washington in 1789. The inauguration date has changed over the years, making this display inaccurate for years before 1937.

Info

George Washington 1789-1797 John Adams 1797-1801 Thomas Jefferson 1801-1809 James Madison 1809-1817 James Monroe 1817-1825 John Quincy Adams 1825-1829 Andrew Jackson 1829-1837 Martin Van Buren 1837-1841 William Henry Harrison 1841 John Tyler 1841-1845 James K. Polk 1845-1849 Zachary Taylor 1849-1850 Millard Fillmore 1850-1853 Franklin Pierce 1853-1857 James Buchanan 1857-1861 Abraham Lincoln 1861-1865 Andrew Johnson 1865-1869 Ulysses S. Grant 1869-1877 Rutherford B. Hayes 1877-1881 James A. Garfield 1881 Chester A. Arthur 1881-1885 Grover Cleveland 1885-1889 Benjamin Harrison 1889-1893 Grover Cleveland 1893-1897 William McKinley 1897-1901 Theodore Roosevelt 1901-1909 William Howard Taft 1909-1913 Woodrow Wilson 1913-1921 Warren G. Harding 1921-1923 Calvin Coolidge 1923-1929 Herbert Hoover 1929-1933 Franklin D. Roosevelt 1933-1945 Harry S. Truman 1945-1953 Dwight D. Eisenhower 1953-1961 John F. Kennedy 1961-1963 Lyndon B. Johnson 1963-1969 Richard Nixon 1969-1974 Gerald Ford 1974-1977 Jimmy Carter 1977-1981 Ronald Reagan 1981-1989 George H. W. Bush 1989-1993 Bill Clinton 1993-2001 George W. Bush 2001-2009 Barack Obama 2009-2017 Donald Trump 2017-2021 Joe Biden 2021-2025 Donald Trump 2025-2029

Accuracy

US terms don't always start on January 20th, with certain stipulations such as if the 20th falls on a Sunday that could change the date slightly. The current system of terms starting on the 20th didn't start until 1937, and previously it was March 4th, with George Washington starting on April 30th.

Source

The data for this entry was sourced from this Wikipedia page.