and Before Christ (abbreviated as BC or B.C.) are designations used to label years in the Julian and Gregorian calendars. The calendar era to which they refer is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, the central figure in Christianity, with AD denoting years after the start of this epoch, and BC denoting years before the start of this epoch.
The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days; and repeats completely every 146,097 days, which fill 400 years. Of these 400 years, 303 (the "common years") have 365 days, and 97 (the leap years) have 366 days.
Anno Hegirae (abbreviated as AH or A.H.)
- The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in many Muslim countries (concurrently with the Gregorian calendar). The first year was the year during which the emigration of the Islamic prophet Muhammad from Mecca to Medina, known as the Hijra, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra). A limited number of years before Hijra (BH) are used to date events related to Islam, such as the birth of Muhammad in 53 BH.
To convert from a Hijri year to a Gregorian year:
G = H + 622 - (H/33)
To convert from a Gregorian year to a Hijri year:
H = G - 622 + ((G - 622)/32)