<Li> The year of farewell . </Li> <P> In 17 AH (638 AD / CE), Abu Musa Ashaari, one of the officials of the Caliph Umar in Basrah, 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 include the date of Muhammad's arrival at Medina (known as Yathrib, before Muhammad's arrival). Uthman ibn Affan 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. Because of the Hijra, the calendar was named the Hijra calendar . </P> <P> The first day of the first month of the Islamic calendar (1 Muharram 1 AH) was set to the first new moon after the day the Prophet moved from Quba' to Medina (originally 26 Rabi' I on the pre-Islamic calendar) i.e., Friday, 16 July 622 AD / 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 AD / 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 . </P> <P> Though Cook and Crone in Hagarism: The Making of the Islamic World cite a coin from 17 AH, the first surviving attested use of a Hijri calendar date alongside a date in another calendar (Coptic) is on a papyrus from Egypt in 22 AH, PERF 558 . </P>

What event marks year one of the islamic calendar
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