The Islamic calendar is based on a lunar year of 12 full lunar cycles, taking exactly 354 and 11/30th days. Each new year in the Islamic calendar thus begins 10 or 11 days earlier in the 364 and one-quarter-day solar calendar. The 12 months of the Islamic year are: Muharram, Safar, Rabi’ al-Awwal (“Rabi’ I”), Rabi’ al-Thani (“Rabi II”), Jumada al-Ula (“Jumada I”), Jumada al-Akhirah (Jumada II), Rajab, Sha’ban, Ramadan, Shawwal, Dhu al-Qa’dah, and Dhy al-Hijjah. The first day of Year One of the Islamic calendar was set as the first day of the hijrah, the Prophet’s migration from Makkah to Madinah on July 26, 622 C.E. The Western convention in designating Islamic dates is thus by the abbreviation AH, which stands for the Latin anno hegirae, or “Year of the Hijrah.”
To roughly convert an Islamic calendar year (AH) into a Gregorian equivalent (A.D./C.E.), or vice versa, use one of the following equations.
AD = 622 + (32/33 x AH)
AH = 33/32 x (AD – 622)