The Islamic Calendar, which is based purely on lunar cycles, was first introduced in 638 CE by the close companion of the Prophet and the second Caliph, Umar ibn Al-KHaTTab (592-644 CE)
The first day of the first month of the Islamic calendar was set to the first new moon after the day the Prophet moved from Quba' to Medina i.e. Friday, 16 July AD 622.
Years are counted since the Hijra Mohammed's emigration to Medina in AD 622.
The Islamic year consists of twelve months. They are: MuHarram, Safar, Raby` al-awal, Raby` al-THaany, Jumaada al-awal, Jumaada al-THaany, Rajab, SHa`baan, RamaDHaan, Shawwal, Thw al-Qi`dah and Thw al-Hijjah.
This Julian date (16 July) was determined by medieval Muslim astronomers by projecting back in time their own tabular Islamic calendar, which had alternating 30- and 29-day months in each lunar year plus eleven leap days every 30 years.