The Muslim era is dated from the hejira, Muhammad's move from Mecca to Medina. In the year 639, Caliph Umar I created a lunar calendar starting with July 16, 622. The years were subsequently numbered A.H. for the Latin Anno Hegirae, "in the year of the Hegira." A little more than a thousand years later, the Ottomans shifted from a lunar to a solar cycle and thereby created a second Hegira calendar with different dates.
The twelve months of the Islamic year are: Muharram, Safar, Rabi' al-Awwal, Rabi' al-Thani, Jumada al-Ula, Jumada al-Akhira, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qa'dah, and Dhu al-Hijjah.
To roughly convert an Islamic calendar year (AH) into a Gregorian equivlent (AD), or vice versa, use one of the following equations:
AD = 622 + (32/33 x AH)
AH = 33/32 x (AD - 622)
Sources: Mitchell G. Bard, The Complete Idiot's Guide to Middle East Conflict. 3rd Edition. NY: Alpha Books, 2005; Saudi Aramco World, (January-February 2002)