The Islamic calendar commenced from the 12 months 622, extra exactly 19 April of the Julian 12 months of 622. It become retrospectively determined in 638 via way of means of Umar bin Khattab, the Islamic caliph at that time, because the 12 months 1 of Hijri era. It become while the Prophet Muhammad and his fans emigrated from Mecca to Medina, called the Hijra.