or Islamic calendar
Dating system used in the Muslim world and based on a year of 12 months, each month beginning approximately at the time of the new moon.
The months are alternately 30 and 29 days long except for the 12th, Dhu al-Hijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the moon. In 11 years of this cycle, Dhu al-Hijjah has 30 days, and in the other 19 years it has 29. Thus the year has either 354 or 355 days. No months are added as in leap years, so the named months do not remain in the same seasons but retrogress through the entire solar, or seasonal, year (of about 365 1 4 days) every 32 1 2 solar years.