There is, however, a consensus among workers in the field that 8 Rabīʿ I was the day of the Hijrah, that ʿUmar instituted the use of the date for the new era, and that this was done in 17.
The choice of the Hijrah as the beginning of the epoch has two reasons.
It is generally accepted that this was done in , “in the year of the Hijrah”).
There are a few points in connection with this that deserve mention: first, there is no real agreement on the exact date of the Hijrah—other dates given include 2 and 12 Rabīʿ I; second, the year in which ʿUmar issued the order is a point of contention—the years 16 and 17 are sometimes given; third, some people have ascribed the use of the chronology to the Prophet himself.
The Didascalia, or the Catholic Teaching of the Twelve Apostles and Holy Disciples of Our Saviour, is a Church Order, composed, according to recent investigations, in the first part, perhaps even the first decades, of the third century, for a community of Christian converts from paganism in the northern part of Syria. A bishop with a considerable knowledge of medicin, he lacked special theological training.
The unknown author of the Didascalia seems to have been of Jewish descent.
According to the Muslim calendar, the Hijrah took place on 8 Rabīʿ I, which corresponds to September 20, 622 (), in the Julian calendar.
Of the alternative chronologies used by Christians, the most important were: (1) the indiction, (2) the Era of Spain, and (3) the Era of the Passion.This chronology was the most widespread in the early Middle Ages, but its use diminished rapidly in the 13th century, although public notaries continued to use it until the 16th.The ), marking the completion of the Roman conquest of Spain.First recorded in the 5th century, it was in general use in Visigothic Spain of the 6th and 7th centuries and, after the Arab invasions, in the unconquered Christian kingdoms in the north of the Iberian Peninsula.It was abolished, in favour of the Islam, Islamic chronology was instituted so soon after the event that was to be the beginning of the Muslim era that no serious problems were encountered in its application.