After Hijra. Muslims reckoned dates from this date.
The Muslim calendar is purely lunar, and it makes no attempt at all to synchronize with the solar calendar (unlike the Chinese, or the pre-Islamic Arabs). Thus, the month of fasting, Ramadan, changes a great deal from year to year in the solar calendar. Thus, if Ramadan falls in summer, for a man living close to fast (literally) from sunrise to sunset is to fast the whole day. In the age before Islam, there were intercalators of the calendar who determine when the sacred months are, and it seems that a synchronizing mechanism was in place before Islam. This was abolished by Muhammad.
622 + H * 354.36 / 365.24
Islam Dictionary Index Start Page
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