A Year Numbered From a Migration, Not a Birth

The first of Muharram opens the Islamic year, and what the date commemorates is not a birth or a revelation but a migration, the Hijra, Muhammad's departure from Mecca to Medina in what corresponds to 622 CE on the Gregorian reckoning. The calendar is not contemporary with the event it starts from. Tradition holds that the caliph Umar ibn al-Khattab, faced with the practical problem of dating administrative documents consistently across an expanding early Islamic state, adopted the Hijra as the calendar's epoch roughly a generation after the migration itself, choosing it over rival candidates such as Muhammad's birth or death.

Because the calendar is purely lunar with no intercalated month, unlike the Hebrew calendar's lunisolar correction, the Islamic year runs about eleven days shorter than the solar year and the new year accordingly cycles slowly backward through every solar season over roughly thirty three years. The tabular reckoning this rule applies is a fixed civil convention, and communities that instead wait for the sighted crescent of Muharram can differ from the table and from each other by a day or more, a variation attributable directly to place rather than to disagreement over the date's meaning.