What calendar is used by Muslims?
Islamic calendar
The Islamic, Muslim, or Hijri calendar ( "at-taqwīm al-hijrī") is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca. The civil calendar of almost all countries where the religion is predominantly Muslim is the Gregorian calendar. Notable exceptions to this rule are Iran and Afghanistan, which use the Solar Hijri calendar. Rents, wages etc. are generally paid by the civil calendar.
List of Islamic years
In principle, each Islamic month begins with sighting of the new crescent moon (after a New Moon) at sunset. Because of this, the calendar is dependent on observational conditions and cannot be predicted or reconstructed with certainty, but tabular calendars are in use which determine the dates algorithmically. Because of this, dates may vary by up to two days between traditions or countries.