stylistic:style_transfer, linguistic:paraphrasing, information:highlighting, structural:scholar_edition
Saudi Arabia's Umm al-Qura calendar

Saudi Arabia has traditionally used the Umm al-Qura calendar, which is based on astronomical calculations, for administrative purposes. The parameters used in the establishment of this calendar underwent significant changes during the decade to AH 1423.

Before AH 1420 (before 18 April 1999), if the moon's age at sunset in Riyadh 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.

From 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 (16 March 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.

In 2007, the Islamic Society of North America, the Fiqh Council of North America and the European Council for Fatwa and Research announced that they would henceforth use a calendar based on calculations using the same parameters as the Umm al-Qura calendar to determine (well in advance) the beginning of all lunar months (and therefore the days associated with all religious observances). This was intended as a first step on the way to unify, at some future time, Muslims' calendars throughout the world.

On 14 February 2016, Saudi Arabia adopted the Gregorian calendar for payment of the monthly salaries of government employees (as a cost cutting measure), while retaining the Islamic calendar for religious purposes.