The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days. It is a lunar calendar having 12 lunar months in a year of about 354 days. Because this lunar year is about 11 days shorter than the solar year, Islamic holy days, although celebrated on fixed dates in their own calendar, usually shift 11 days earlier each successive solar year, such as a year of the Gregorian calendar. Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred— Muhammad's emigration from Mecca to Medina. Thus each numbered year is designated either H or AH, the latter being the initials of the Latin anno Hegirae (in the year of the Hijra).
The official Umm al-Qura calendar of Saudi Arabia used a substantially different astronomical method until recent years *. Before AH 1420 (before April 18, 1999), if the moon's age at sunset in Riyad was at least 12 hours, then the day ending at that sunset was the first day of the month. This often caused the Saudis to celebrate holy days one or even two days before other predominantly Muslim countries, including the dates for the Hajj, which can only be dated using Saudi dates because it is performed in Mecca. During one memorable year during the AH 1380s (the 1970s), different Muslim countries ended the fast of Ramadan on each of four successive days. The celebrations became more uniform beginning in AH 1420. For AH 1420-22, if moonset occurred after sunset at Mecca, then the day beginning at that sunset was the first day of a Saudi month, essentially the same rule used by Malaysia, Indonesia, and others (except for the location from which the hilal was observed). Since the beginning of AH 1423 (March 16, 2002), the rule has been clarified a little by requiring the geocentric conjunction of the sun and moon to occur before sunset, in addition to requiring moonset to occur after sunset at Mecca. This ensures that the moon has moved past the sun by sunset, even though the sky may still be too bright immediately before moonset to actually see the crescent. Strictly speaking, the Umm al-Qura calendar is intended for civil purposes only. Their makers are well aware of the fact that the first visual sighting of the lunar crescent (hilāl) can occur up to two days after the date calculated in the Umm al-Qura calendar. Since AH 1419 (1998/99) several official hilāl sighting committees have been set up by the government of Saudi Arabia to determine the first visual sighting of the lunar crescent at the begin of each lunar month. Nevertheless, the religious authorities of Saudi Arabia also allow the testimony of less experienced observers and thus often announces the sighting of the lunar crescent on a date when none of the official committees could see the lunar crescent. In nearly all of these cases, a retrospective analysis indicates that these extremely early reports of the lunar crescent are impossible and are based on false sightings.
The moon sets progressively later than the sun for locations further west, thus western Muslim countries are more likely to celebrate some holy day one day earlier than eastern Muslim countries.
Microsoft uses the "Kuwaiti algorithm" to convert Gregorian dates to the Islamic ones. It is claimed to be based on a statistical analysis of historical data from Kuwait but it is in fact a variant of the tabular Islamic calendar discussed in the next paragraph.
There exists a variation of the Islamic calendar known as the tabular Islamic calendar in which months are worked out by arithmetic rules rather than by observation or astronomical calculation. It has a 30-year cycle with 11 leap years of 355 days and 19 years of 354 days. In the long term, it is accurate to one day in about 2500 years. It also deviates up to about 1 or 2 days in the short term.
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.
Verily the transposing (of a prohibited month) is an addition to Unbelief: The Unbelievers are led to wrong thereby: for they make it lawful one year, and forbidden another year, of months forbidden by Allah and make such forbidden ones lawful. The evil of their course seems pleasing to them. But Allah guideth not those who reject Faith.
This prohibition was repeated by Muhammad during his last sermon on Mount Arafat which was delivered during his farewell pilgrimage to Mecca on 9 Dhu al-Hijja AH 10 (this paragraph is often deleted from the sermon by its modern editors as now unimportant):
O People, the unbelievers indulge in tampering with the calendar in order to make permissible that which Allah forbade, and to forbid that which Allah has made permissible. With Allah the months are twelve in number. Four of them are holy, three of these are successive and one occurs singly between the months of Jumada and Shaban.
The three successive holy months are Dhu al-Qada, Dhu al-Hijja, and Muharram, thus excluding an intercalary month before Muharram. The single holy month is Rajab.
The Islamic months are named as follows:
Of all the months in the Islamic calendar, Ramadan is the most sacred. Between dawn and sunset, Muslims abstain from eating, drinking, and sexual intercourse in accordance with the Ramadan holiday that lasts throughout the entire month of the same name.
The Islamic calendar year of 1429 will occur entirely within the Gregorian calendar year of 2008. Such years occur once every 33 or 34 Islamic years (32 or 33 Gregorian years).
More are listed here:
| Islamic year within Gregorian year | ||
| Islamic | Gregorian | Difference |
| 1228 | 1813 | 585 |
| 1261 | 1845 | 584 |
| 1295 | 1878 | 583 |
| 1329 | 1911 | 582 |
| 1362 | 1943 | 581 |
| 1396 | 1976 | 580 |
| 1429 | 2008 | 579 |
| 1463 | 2041 | 578 |
| 1496 | 2073 | 577 |
| 1530 | 2106 | 576 |
| 1564 | 2139 | 575 |
تقويم هجري | Ислямски календар | Calendari musulmà | Islámský kalendář | Islamisk kalender | Islamische Zeitrechnung | Islami kalender | Calendario musulmán | Islama kalendaro | Calendrier musulman | Calendario musulmán | Islama kalendario | Kalender Hijriyah | Íslamska tímatalið | הלוח המוסלמי | Kalenda ya Kiislamu | Takwim Hijrah | Islamitische kalender | ヒジュラ暦 | Det muslimske året | Kalendarz muzułmański | Calendário islâmico | Исламский календарь | Muslimanski koledar | Islamilainen kalenteri | Muslimska kalendern | Íslam täqwime | ปฏิทินฮิจญ์เราะหฺ | Hicri takvim | 伊斯兰历
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"Islamic calendar".
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