Islamic calendar
Year numbering

In pre-Islamic Arabia, it was customary to identify a year after a major event which took place in it. Thus, according to Islamic tradition, Abraha, governor of Yemen, then a province of the Christian Kingdom of Aksum of Northeast Africa and South Arabia, attempted to destroy the Kaaba with an army which included several elephants. The raid was unsuccessful, but that year became known as the Year of the Elephant, during which Muhammad was born (surah al-Fil). Most equate this to the year 570 CE, but a minority use 571 CE.

The first ten years of the Hijra were not numbered, but were named after events in the life of Muhammad according to al-Biruni:

In c. 638 (17 AH), Abu Musa al-Ash'ari, one of the officials of the Rashid Caliph Umar (r. 634–644) in Basra, complained about the absence of any years on the correspondence he received from Umar, making it difficult for him to determine which instructions were most recent. This report convinced Umar of the need to introduce an era for Muslims. After debating the issue with his counsellors, he decided that the first year should be the year of Muhammad's arrival at Medina (known as Yathrib, before Muhammad's arrival). Uthman then suggested that the months begin with Muharram, in line with the established custom of the Arabs at that time. The years of the Islamic calendar thus began with the month of Muharram in the year of Muhammad's arrival at the city of Medina, even though the actual emigration took place in Safar and Rabi' I of the intercalated calendar, two months before the commencement of Muharram in the new fixed calendar. Because of the Hijra, the calendar was named the Hijri calendar.

F A Shamsi (1984) postulated that the Arabic calendar was never intercalated. According to him, the first day of the first month of the new fixed Islamic calendar (1 Muharram AH 1) was no different from what was observed at the time. The day the Prophet moved from Quba' to Medina was originally 26 Rabi' I on the pre-Islamic calendar. 1 Muharram of the new fixed calendar corresponded to Friday, 16 July 622 CE, the equivalent civil tabular date (same daylight period) in the Julian calendar. The Islamic day began at the preceding sunset on the evening of 15 July. This Julian date (16 July) was determined by medieval Muslim astronomers by projecting back in time their own tabular Islamic calendar, which had alternating 30- and 29-day months in each lunar year plus eleven leap days every 30 years. For example, al-Biruni mentioned this Julian date in the year 1000 CE. Although not used by either medieval Muslim astronomers or modern scholars to determine the Islamic epoch, the thin crescent moon would have also first become visible (assuming clouds did not obscure it) shortly after the preceding sunset on the evening of 15 July, 1.5 days after the associated dark moon (astronomical new moon) on the morning of 14 July.

Though Michael Cook and Patricia Crone in their book Hagarism cite a coin from AH 17, the first surviving attested use of a Hijri calendar date alongside a date in another calendar (Coptic) is on a papyrus from Egypt in AH 22, PERF 558.
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