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.