The Islamic calendar, adopted formally under Caliph Umar around 638 CE, was set to begin its count from the year of the Hijra, Muhammad's migration from Mecca to Medina in 622 CE. This lunar calendar, roughly eleven days shorter than the solar year, governs Islamic religious observance worldwide, including Ramadan and the Hajj pilgrimage, and its epoch date is conventionally reckoned to July 16, 622 in the proleptic Julian calendar.
The Hijra, Muhammad's flight with his followers from persecution in Mecca to the more hospitable city of Medina (then Yathrib), marked a turning point in early Islamic history. In Medina, Muhammad became not just a religious leader but a political and military one, uniting warring tribes under a new social and legal order that would eventually spread across Arabia.
The calendar itself was formalized years later, during the caliphate of Umar ibn al-Khattab, who needed a standardized dating system for administering the rapidly expanding Islamic state. Rather than starting the count from Muhammad's birth or the year of revelation, Umar and his advisors chose the Hijra as the founding moment, a decision that emphasized the migration and community-building over the individual prophet.
Astronomers later calculated the epoch date, the first day of the first month of year one, as corresponding to July 16, 622 CE in the Julian calendar (July 19 in the proleptic Gregorian calendar), even though the actual migration occurred later in the year; the calendar's start was set retroactively to the beginning of that Arabic year. Today, the Hijri calendar remains the official calendar in Saudi Arabia and governs Islamic religious life for over a billion Muslims worldwide.