16 July 622 is believed to be the beginning of the Islam calendar. Just as the Christian calendar places, or tried to place, year zero at the time of Christ. Muslim scholars have tried to put year zero and the time of Mohammad’s move from Quba’ to Medina.
This Julian date (16 July) was determined by medieval Muslim astronomers by projecting back in time their own tabular Islamic calendar. Al-Biruni mentioned this date in the year 1000 AD/CE.