The Hijri Calendar has years marked by AH, which stands for Anno Hegirae, i.e. "In the Year of the Hijra"
The calendar system which is based on the Earth’s revolving around the Sun, and marks the arrival day (September 20, 622) of our Prophet (PBUH) in Quba (masjid) as the beginning is named Hijri Solar Calendar. In the pre-Islamic period every important event used to be accepted as the beginning of an era. Ultimately the Elephant Issue had been accepted as the beginning date. In this practice, since the years were counted according to important events, it would lead to much confusion.
The Hijri calendar is the Islamic dating system deriving its name from the Hijra, the Arabic word for migration, referring to the migration of the prophet Muhammad (PBUH) from Mecca to Yathrib (Medina) in the Arabian Peninsula in 622 A.D. thus marking the first year in the Islamic calendar. The first day of the Islamic calendar, 1 Muharram, 1 A.H. (Anno Hijrae) is Friday July 16, 622 A.D Julian.
At the 17th year of Hijra (migration) a decision was made that the year Hijra was accepted as the first year of Hijri Calender, and the date July 16, 622- with which Muharram 1st coincided in that yearwas accepted as the beginning of Hijri Lunar Calendar in the period of Hazrat ‘Umar (RA). We know it as Hijri Calendar.
In Hijri Lunar Calendar the months are as follows: Muharram, Safar, Rabii’ I, Rabii’ II, Jumada I, Jumada II, Rajab, Sha’aban, Ramadan, Shawwal, Dhu al-Qa’ada, Dhu al-Hijja.
There are leap years in Hijri Calendar as in Gregorian calendar. It drifts nearly 11 days in 30 years.
In order to correct this drift the years 2, 5, 7, 10, 13, 15, 18, 21, 24, 26 and 29 are 355 days, and the rest are 354 days in every 30 years period.
One Hijri lunar year comprises twelve full cycles of the Moon around the Earth, and it is 354,367 days (354 days, 8 hours, 48 minutes and 34,690 seconds.) One Gregorian year comprises one full cycle of the Earth around the Sun, and it is 365,2422 days. Since Hijri year is 10,8752 days shorter than Gregorian year, the months sometimes last 29, and sometimes 30 days.