In A.D. 638, the second Caliph Umar ibn al-Khattab (A.D. 592-644) introduced the calendar as a way of consolidating the various calendars then in common usage among Muslim peoples. The years are measured from the first of Muharram of the year in which Muhammad emigrated to the city of Medina, which corresponds to July 16, A.D. 622 (the actual emigration took place in September). The calendar is also called the Hijri Calendar as this migration is called the Hegira. The first day of the first month (1 MuHarram) of the first year (1 AH) is this day. Dates in this calendar are usually abbreviated using AH from the latinized phrase Anno Hegirae, "in the year of the Hegira". Years before the Hegira are denoted BH: there is no year zero.
The predecessor to the Islamic calendar was similar to the Hebrew calendar in that it was primarily lunar but was kept synchronized with the tropical year (that is, based on the motion of the sun) by the insertion of an additional month when required. In the 9th year after the Hejira or migration to Medina, Muhammad forbade the insertion of the additional months.
This is expressed in the 9th chapter and 37th verse of the Quran as:
Of all the months in the Islamic calendar, Ramadan is the most sacred, and all Muslims are required to fast during the daytime.
The Islamic months are named as follows:
The names of the days of the week
The number of days in each month is not set. Rather, each month begins at sunset on the day of the first sighting of the lunar crescent following a new moon. Traditionally, this requires a sighting by a human observer, and this practice is still followed in many parts of the world. In some countries near the Persian Gulf, the process is simplified by beginning each month at sunset on the first day that the moon sets after the sun. In Egypt, the month begins at sunset on the first day that the moon sets at least five minutes after the sun. These simplifications allow the calendar to be determined in advance, which it cannot be by the traditional method.
Microsoft uses the "Kuwaiti algorithm" to convert Gregorian dates across the Islamic ones. It is based on statistical analysis of historical data from Kuwait.
The Holy Quran, in the fifth chapter and 36th verse mentions the calendar (translated into English) states: "The number of months with Allah has been twelve months by Allah's ordinance since the day He created the heavens and the earth. Of these four are known as sacred; That is the straight usage, so do not wrong yourselves therein, and fight the Pagans. "
These four sacred months are: Muharram, Rajab, Dhul Qadah and Dhul Hijja. It is from this verse that it is commonly believed that fighting during sacred months is a sin.
Extremely important dates in the Islamic (Hijri) year are:
The Islamic calendar year of 1429 occurs entirely within the Gregorian calendar year of 2008. Such years occur once every 33 or 34 Islamic years (32 or 33 Gregorian years).