The Hijri year (Arabic: سَنة هِجْريّة) or era (التقويم الهجري at-taqwīm al-hijrī) is the era used in the Islamic lunar calendar, which begins its count from the Islamic New Year in 622 CE. During that year, Muhammad and his followers migrated from Mecca to Yathrib (now Medina). This event, known as the Hijra, 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 ˈhɛdʒᵻriː/, "in the year of the Hijra") in parallel with the Christian (AD) and Jewish eras (AM) and can similarly be placed before or after the date. In Muslim countries, it is also commonly abbreviated H ("Hijra") from its Arabic abbreviation hāʾ (هـ). Years prior to AH 1 are reckoned in English as BH ("Before the Hijra"), which should follow the date.
Because the Islamic lunar calendar has only 354 days in its year, it slowly rotates within the Gregorian year. The year 2017 CE corresponds to the Islamic years AH 1438 – 1439.
The Hijri era is calculated according to the Islamic lunar calendar and not the Julian or Gregorian solar one. It thus does not begin on January 1, 1 CE, but on the first day of the month of Muharram which occurred in 622 CE. Its Julian equivalent was April 19 but it is sometimes mistakenly placed on July 16. The error derives from the tabular Islamic calendar which was devised by later Islamic astronomers. This reckons time backwards according to the lunar calendar, which causes it to miss the three intercalary months (about 88 days) added to the then-lunisolar calendar between the time of the Hijra and AH 10, when Muhammad is recorded as having received a revelation prohibiting their use.
The date of the Hijra itself did not form the Islamic New Year. Instead, the system continues the earlier ordering of the months with the Hijra occurring around the 8th day of Rabi al-Awwal, 66 days into the first year.
By the age of Muhammad, there was already an Arabian lunar calendar with named months. The years of its calendar, however, used conventional names rather than numbers: for example, the year of Muhammad and Ammar ibn Yasir's birth (570 CE) was known as the "Year of the Elephant". The year of the Hijra (622-23 CE) was initially named the "Permission to Travel".
17 years after the Hijra, a complaint from Abu Musa Ashaari prompted the caliph Umar to abolish the practice of named years and to establish a new calendar era. Rejected proposals included dating from the year of Muhammad's birth or death. Tradition credits ʿAli with the proposal to date from the year during which the Muslims established a new community (Ummah) in Medina. The order of the months within the calendar was then debated. Rejected proposals included Rajab, which had been a sacred month in the pre-Islamic period; Ramadan, which is a sacred month for Muslims; and Dhu al-Hijjah, the month of the Hajj. Tradition credits Usman with the successful proposal, simply continuing the order of the months that had already been established, beginning with Muharram. Adoption of this calendar was then enforced by Umar.
Different approximate conversion formulas between the Gregorian (AD or CE) and Islamic calendars (AH) are possible:AH = 1.030684 × (CE − 621.5643) CE = 0.970229 × AH + 621.5643
orAH = (CE − 622) × 33 ÷ 32 CE = AH + 622 − (AH ÷ 32)
Nevertheless, as the Islamic year does not begin 1 January there is no strict correspondence between years of the two eras, e. g. 2015 CE is 1436/1437 AH, while 1436 AH is 2014/2015 CE.