Hijri era

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The Hijri era is the Islamic calendar system used to date events in the Muslim world. It begins with the Hijrah, Muhammad’s migration from Mecca to Medina in 622 CE, which marks the start of the first Muslim community.

There are two main calendars that count years from this moment:
- The lunar Hijri calendar (AH): Used by most Muslims. It has 12 lunar months and a year of about 354 or 355 days. Because lunar years are shorter, the Islamic New Year moves about 10 days earlier each Gregorian year. Years are written AH (Anno Hegirae). Before the Hijrah, years are written BH (Before the Hijra).
- The solar Hijri calendar (SH): Used in Iran. It counts solar years from the Hijrah and begins in the March equinox, so it matches the seasons closely. Years are written SH.

Important notes:
- The Hijrah year starts on the first day of Muharram in most contexts, but some traditions (notably Twelver Shia) begin the year in a different month (Rabi’ al-Awwal). This can lead to slight differences in dating some events.
- Because lunar years are shorter than solar years, there is no exact one-to-one mapping with the Gregorian calendar. A given Hijri year typically spans parts of two Gregorian years.
- The solar Hijri year is very close in length to the Gregorian year, and its dates align with the March equinox rather than January 1.

Summary:
The Hijri era anchors Islamic dates to the Hijrah of 622 CE and is counted in two ways: the lunar AH calendar used widely across the Muslim world, and the solar SH calendar used in Iran. BH refers to years before the Hijrah. The two calendars run on different year lengths, so they line up with the Gregorian calendar in different ways.