The Islamic calendar was dated to start with the first new moon after the Prophet Muhammad and his followers migrated to Medina.
The calendar begins with the Prophet’s Flight as this is a key event in Islamic history for which all early followers could agree on the specific date. There was disagreement over the exact date of other events, such as the birth of the Prophet or when he first received the Divine message.
Known as the Hijra the Prophet Muhammad led his followers from Mecca to Medina, which at the time was called Yathrib, due to rumours of an assassination plot against him. Despite the migration taking place in 622, this year was only set as the start of the Islamic calendar by the Caliph Umar in 638 due to the pressing need to have a formalised dating system to improve administration. Until then the Muslim community had identified years according to a key event that took place within it – such as ‘the year of permission’.
While the Islamic calendar is linked to the Hijra, the actual start date is based on the beginning of the month of Muharram in the year of the Prophet’s arrival in Medina. This lunar month was already important to pre-Islamic Arabs and so served as a sensible demarcation, especially as it had been named by Allah in the Quran as one of the four sacred months.
16 July 622 was only identified on the Western Julian calendar during the medieval period. Muslim astronomers created a tabular Islamic calendar that they then projected backwards to identify the equivalent date on the Julian calendar. A tabular Islamic calendar relies on arithmetical rules to determine the length of the months, rather than astronomical calculations. Lunar observations are still used to specify the correct date of Islamic holidays and rituals.