17 years after the Hijra, a complaint from Abu Musa Ashaari prompted the caliph Umar to abolish the practice of named years and to establish a new calendar era. Umar chose as epoch for the new Muslim calendar the hijrah, the emigration of Muhammad and 70 Muslims from Mecca to Medina. Tradition credits Othman with the successful proposal, simply continuing the order of the months that had already been established, beginning with Muharram. Adoption of this calendar was then enforced by Umar.
Different approximate conversion formulas between the Gregorian (AD or CE) and Islamic calendars (AH) are possible:
AH = 1.030684 × (CE − 621.5643) CE = 0.970229 × AH + 621.5643
AH = (CE − 622) × 33 ÷ 32 CE = AH + 622 − (AH ÷ 32)