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.