How did the Rashidun caliphs establish the Islamic calendar?

The establishment of the Islamic calendar is attributed to the second Rashidun caliph, ‘Umar I, around 639 CE (17 AH).12

The need for a formal dating system arose when the caliph’s attention was drawn to the fact that his official correspondence lacked dates. Following consultations with his advisers in Medina, ‘Umar decided that the Hijrah—the Prophet Muhammad’s migration from Mecca to Medina—should serve as the epoch for the Muslim state and community. This event was chosen because it marked the transformation of Islam into both a religion and a state.21

While the migration itself occurred in the month of Rabī‘ I, ‘Umar ordered that the new era begin with the first day of the lunar month of Muḥarram, corresponding to July 16, 622 CE. The resulting Islamic calendar utilized a lunar year, a practice consistent with existing Arabian traditions. This system provided a unified chronological framework that strengthened the Muslim community and facilitated religious and administrative record-keeping.123