Why does the Islamic calendar begin with the Hijrah in 622?

The Islamic calendar begins with the Hijrah—the migration of the Prophet Muhammad from Mecca to Medina in 622 CE—because this event marked a pivotal transformation in Islamic history. By migrating, Muhammad and his followers established the Muslim community as a distinct sociopolitical entity for the first time, effectively allowing Islam to function as both a religion and a state.12

The use of the Hijrah as the starting point for the Islamic calendar was formally instituted by the second caliph, ‘Umar I, around 639 CE. Recognizing the importance of the migration, ‘Umar and his advisers chose this era because its date was historically fixed and it represented the foundational moment of the Muslim polity. While the migration itself occurred in the month of Rabī‘ I, ‘Umar ordered that the calendar year begin with the first day of the lunar month of Muḥarram, which corresponded to July 16, 622 CE, in the Julian calendar.32