The Hijri year (Arabic: سَنة هِجْريّة) or era (التقويم الهجريat-taqwīm al-hijrī) is the era used in the Islamic lunar calendar, which begins its count from the Islamic New Year in 622 AD/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 Hegiraeiː/, "in the year of the Hijra") in parallel with the Christian (AD), Common (CE) 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 or 355 days in its year, it slowly rotates within the Gregorian year. The year 2018 AD corresponds to the Islamic years 1439 – 1440 AH.
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.