which event marks the start of the islamic calendar
Islamic calendar The Islamic calendar employs the Hijri era whose epoch was retrospectively established as the Islamic New Year of AD 622. During that year, Muhammad and his followers migrated from Mecca to Yathrib (now Medina) and established the first Muslim community (ummah), an event commemorated as the Hijra. In the West, dates in this era are usually denoted AH (Latin: Anno Hegirae, "in the year of the Hijra") in parallel with the Christian (AD) and Jewish eras (AM). In Muslim countries, it is also sometimes denoted as H[1] from its Arabic form ( سَنة هِجْريّة, abbreviated هـ). In English, years prior to the Hijra are reckoned as BH ("Before the Hijra").[2]
History of calendars While the Gregorian calendar is now in worldwide use for secular purposes, various medieval or ancient calendars remain in regional use for religious or social purposes, including the Julian calendar, the Hebrew calendar, the Islamic calendar, various Hindu calendars, the Zoroastrian calendar etc.