Which calendar system is used officially in Iran for dating Nowruz?

Iran officially uses the Solar Hijri calendar for dating Nowruz. This calendar counts solar years from the Hijra, the migration of Muhammad from Mecca to Medina, while placing the New Year at the March equinox.

The modern Iranian calendar has twelve months and is highly accurate because the beginning of the year is tied to the astronomical equinox. The first six months have thirty-one days, the next five have thirty days, and the final month has twenty-nine or thirty days depending on the year.

The Gregorian calendar is widely used internationally and is often shown alongside Iranian dates, but it does not determine the Iranian New Year. The Solar Hijri calendar is also called the Iranian calendar or Persian calendar in many English sources, although historical Persian calendars include earlier systems with different rules.