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Who introduced the Hijrah era for the Muslim calendar?
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Muhammad
The Prophet's Mosque, showing the green dome built above the tomb of Muhammad, Medina, Saudi Arabia.
The Hijrah era for the Muslim calendar was introduced by ‘Umar I, the second caliph, in 639 CE (AH 17).123
After his attention was drawn to the fact that his correspondence lacked dates, ‘Umar consulted with his advisers in Medina and ordered that the year of the Hijrah—the Prophet Muhammad’s migration from Mecca to Medina—be established as the beginning of the era for the Muslim state and community. Although the migration itself occurred in the month of Rabī‘ al-Awwal, ‘Umar decreed that the first year of the era should begin with the first day of the lunar month of Muḥarram, which corresponded to July 16, 622 CE, in the Julian calendar.312