Answer: the Prophet Muhammad’s migration (622 CE) from Mecca to Medina in order to escape persecution. The date represents the starting point of the Muslim era. Muhammad himself dated his correspondence, treaties, and proclamations after other events of his life. It was ʿUmar I, the second caliph, who in the year 639 CE introduced the Hijrah era (now distinguished by the initials AH, for Latin anno Hegirae, “in the year of the Hijrah”). ʿUmar started the first year AH with the first day of the lunar month of Muḥarram, which corresponds to July 16, 622, on the Julian calendar. In 1677–78 (AH 1088) the Ottoman government, still keeping the Hijrah era, began to use the solar (Julian) year, eventually creating two different Hijrah era dates, resulting from the differences between a solar and a lunar year.
