The Hijri year is the era use in to the Islamic lunar calendar, which begin its tally from Islamic New Year in 622 CE. Throughout that year, Muhammad and his followers migrate as of Mecca to Yathrib now its name Medina. This occasion, recognized as the Hijra, is commemorate in Islam for its position in the beginning of the first Muslim group of people (ummah).
In the West, this period is most usually denote as AH “Anno Hegirae” in the year of the Hijra similar with the Christian and Jewish eras and can also be placed before or after the date. In Muslim countries it is also usually shortened H (“Hijra”) from its Arabic short form in Years previous to AH are reckon in English BH “Before the Hijra” which should go after the date.
Because the Islamic astral calendar has just 354 days in its year, it slowly rotate within Gregorian year.