The Islamic Calendar, which is based purely on lunar cycles, was first introduced in 638 C.E. by the close companion of the Prophet (peace be upon him) and the second Caliph, `Umar ibn Al-Khattab (592-644 C.E.). He did it in an attempt to rationalize the various, at times conflicting, dating systems used during his time.

To Muslims, the Hijri calendar is not just a sentimental system of time reckoning and dating important religious events, e.g., Siyam (fasting) and Hajj (pilgrimage to Makkah). It has a much deeper religious and historical significance. It is a unique occasion to ponder that the Islamic Era did not start with the victories of Islamic wars, nor with the birth or death of the prophet (peace be upon), nor with the revelation of the Quran. It starts with Hijra, or the sacrifice for the cause of Truth and for the preservation of the Revelation.

Using different formulas reflects different reasons. Christians commemorate the birth of Jesus while Muslims start their calendar by the sacrifice they did for the prosperity of their religion, i.e. the beginning of their immigration from Mecca to Medina.