Abbreviation for the Latin phrase Anno Hejira, "in the year of the Hejira," used to refer to the year on the Islamic calendar.
The year 1 A.H. occured in the year 622 C.E., but unfortunately, converting between the two systems is not a matter of simple subtraction. The Islamic calendar is lunar, which means that the year is only 354 days long (or 355 days in certain years).