The modern Hijri calendar is based on and uses the same month names as the pre-Islamic calendar used by the Ancient Arabs. According to some sources, it was a lunisolar calendar where the length of the months followed the Moon phases and a leap month (Nasī’) was regularly inserted to re-align the time reckoning with the solar year. Following the Muslim conquest of Makkah, the leap month was abolished to create a purely lunar calendar.

The Iranian astronomer Al-Biruni (973 – 1048 CE) states that the Caliph Umar (c.583 – 644 CE) introduced the Islamic year count in 638 CE. Pre-Islamic time reckoning did not utilize year numbers as each year was identified by an important event that occurred in it.