The Hijra, or Muslim era started on the 1st of Muharram of the year 1, corresponding to the 16th of July of year 622 of the Christian era.

Hijra, means in Arabic departure or migration, becasue it marked the day on which the Prophet Mohammed left Mecca to take refuge in Yathrib, a city which was later called Medina (Medinat al-Nabi, the city of the Prophet). The califa Omar definitely fixed, in 637, the beginning of the Muslim era.

Since the Muslim lunar calendar has 11 days less than the Christian calendar, it is quite difficult to put both calendars in agreement with one another. To add or substract 622 gives only an approximation: 1999 AC corresponds to the Hijra years 1419/420…