It begins its count from the Islamic New Year in which Muhammad and his followers migrated from Mecca to Yathrib (now Medina) in 622 CE.
This event, known as the Hijrah, is commemorated in Islam for its role in the founding of the first Muslim community (ummah).
In the West, this era is most commonly denoted as AH (Latin: Anno Hegirae, /ˈænoʊ ˈhɛdʒɪriː/, lit.
Years prior to AH 1 are reckoned in English as BH ("Before the Hijra"), which should follow the date.
[8] Tradition credits Othman with the successful proposal, simply continuing the order of the months that had already been established by Muhammad, beginning with Muharram,[citation needed] as there was no set order of months during the pre-Islamic era (Age of Ignorance – Jahiliya).
The Hijri year has twelve months, whose precise lengths vary by sect of Islam.
Consequently, each month can have 29 or 30 days depending on the visibility of the moon, astronomical positioning of the earth and weather conditions.