Some time in 638 AD, Abu Musa Asha'ari, the Governor of Basra wrote:
"Amir-ul-Mominin, we receive instructions from you every now and then, but as the letters are undated, and some times the contents of the letters differ, it becomes difficult to ascertain as to which instructions are to be followed."
That set Umar thinking. In the meantime, he received from Yemen a draft for some money which was encashable in Shaban. Umar thought that the practice of merely mentioning the month in such cases was defective for one could not be sure whether the month referred to was of the current or the following year.
Umar convened an assembly to consider the question of calendar reform.
Some one suggested that the Roman calendar should be adopted. After discussion the proposal was rejected as the Roman calendar dated from too remote an era and was cumbersome.
It was next considered whether the Persian calendar might be adopted. Hormuzan explained the salient features of the Persian calendar called 'Mahroz'. The consensus of opinion was that such a calendar would not be suitable for the Muslims.
The general opinion was that instead of adopting any alien calendar, the Muslims should have a calendar of their own. This was agreed to, and the point next considered was from when should such an era begin?
Some one suggested that the era should begin from the date of birth of the Holy Prophet. Some suggested that it should begin from the death of the Holy Prophet. Ali suggested that it should begin from the date the Muslims migrated from Mecca to Madina. After discussion, Ali's suggestion was agreed to.
The Holy Prophet had migrated in the month of Rabi-ulAwwal, when the year had already run two months and eight days. Next the question arose from which month should the new era start.
Some one suggested that the calendar should start with the month of Rajab as in the pre-Islamic period this month was held sacred. Some one proposed that the first month should be Ramzan as that is a sacred month for the Muslims. Another proposal was that the first month should be 'Zul Hajj' as that is the month of the pilgrimage.
Usman suggested that as in Arabia the year started with Muharram the new era should also start with Muharram. This suggestion was accepted. The date was accordingly pushed back by two months and eight days, and the new Hijri calendar began with the first day of Muharram in the year of migration rather than from the actual date of migration.
Umar accordingly issued instructions to all concerned regarding the enforcement of the Hijri calendar.