The Umm al-Qura Calendar Explained: How Saudi Dates Are Set

The Umm al-Qura calendar is the calculated Hijri calendar of Saudi Arabia. Its dates are worked out in advance from the positions of the sun and moon as seen from Makkah, not from a sighting of the crescent. Under the rule used since 1423 AH (2002), a month ends after 29 days only if, on the evening of the 29th, the new moon has already happened before sunset and the moon sets after the sun. Otherwise the month runs to 30 days. The calendar serves civil purposes; the start of Ramadan, Eid and the Hajj month in Saudi Arabia is still announced after reported sightings, and those announcements can differ from the printed date.

Key Takeaways

• The Umm al-Qura calendar is a calculated lunar calendar, computed for the location of the Ka'ba in Makkah.
• Since 1423 AH, a new month begins after the 29th only if conjunction comes before sunset and moonset comes after sunset at Makkah.
• The rules have changed several times since the 1970s, so old Umm al-Qura dates follow older rules.
• It is a civil calendar. Religious months in Saudi Arabia can still be adjusted after crescent sightings, by a day in either direction.

What the Umm al-Qura calendar is

The name means "Mother of Towns," a title for Makkah. It comes from the Quran, where Surah al-An'am 6:92 speaks of warning Umm al-Qura and those around it; The Clear Quran translation on quran.com renders it "the Mother of Cities."

The calendar is tied to that city literally. R.H. van Gent of Utrecht University, whose pages are the most detailed English-language account of the calendar, notes in his introduction to the Umm al-Qura calendar that the Ka'ba in the Great Mosque of Makkah defines the latitude and longitude for which it is calculated. He also notes that neighbouring states such as Bahrain, Qatar and the United Arab Emirates follow it.

The dates are now computed at the Institute of Astronomical and Geophysical Research of the King Abdulaziz City for Science and Technology (KACST) in Riyadh, according to van Gent's page on the astronomical rules of the Umm al-Qura calendar. The same page describes it as the calendar "used by the government of Saudi Arabia for civil purposes."

Because the dates are fixed by calculation, the calendar can be published years ahead. Software uses it too: Microsoft's .NET documentation for the UmAlQuraCalendar class says it uses "a table-based algorithm licensed from the Saudi government" and supports dates up to 1500 AH (November 2077).

The current rule, in use since 1423 AH

The two conditions

Since the start of 1423 AH (15 March 2002), each month is decided on its 29th day. Van Gent's rules page gives the two conditions:

1. The geocentric conjunction (the astronomical new moon) occurs before sunset.
2. The moon sets after the sun.

If both are true at Makkah on the 29th, the next day is the 1st of the new month. If either fails, the month lasts 30 days.

The International Astronomical Center's page on the Saudi dating system reports the same two conditions, attributing them to Dr. Zaki al-Mostafa of KACST.

This rule guarantees two things: the month never starts before the new moon, and the moon is above the horizon at sunset in Makkah on the first evening. It does not guarantee that anyone can see the crescent. Van Gent estimates that in about 75% of cases the crescent is not visible to the naked eye on the first evening, because the moon is often only just above the horizon.

Worked example: a 30-day month in 1448 AH

Monday 9 November 2026 is 29 Jumada al-Awwal 1448 in the calculated Umm al-Qura calendar. The US Naval Observatory's moon phases data and sun and moon rise/set data give these times for that day, for the Ka'ba's coordinates (21.4225° N, 39.8262° E) in Makkah time (UTC+3):

The first condition is met, but the second is not. So Jumada al-Awwal runs to 30 days. In the calculated calendar, Tuesday 10 November 2026 is 30 Jumada al-Awwal, and Wednesday 11 November is 1 Jumada al-Thani 1448. Because the Islamic day begins at sunset, 1 Jumada al-Thani starts at sunset on Tuesday evening.

These are USNO figures, rounded to the minute, not KACST's own published computation. The Umm al-Qura conversion used for the dates above also gives Jumada al-Awwal 1448 thirty days.

Worked example: a 29-day month that sighting did not confirm

The opposite case shows why a calculated month can start before anyone sees the crescent. In a prediction note for Muharram 1435, Dr. Ali Al-Shukri of the physics department at King Fahd University of Petroleum and Minerals (KFUPM) computed that on Sunday 3 November 2013 (29 Dhu al-Hijjah 1434) the conjunction came at 15:51 Saudi time and the moon set about 21 seconds after the sun at Makkah.

Both conditions were met, so the Umm al-Qura calendar began Muharram on Monday 4 November. The same note judged a sighting impossible that evening and expected the month to begin on Tuesday 5 November by sighting. Van Gent's comparison table records the announced 1 Muharram 1435 as 5 November 2013, one day after the calculated date.

How the rules changed over time

The Umm al-Qura calendar has not always used the 1423 AH rule. Van Gent reconstructs four periods:

The start date of 1 Muharram 1392 AH (16 February 1972) comes from the Umm al-Qura conversion; van Gent's rules page prints "16 March 1972" for it, which does not match the calendar's own dates. Sources also do not describe the older rules in identical terms. The International Astronomical Center's page, for example, describes the pre-1420 rule in terms of the moon's age at sunset rather than a midnight cut-off. Both accounts agree on the 1420 change and the current 1423 rule.

The practical point for anyone converting older dates: an Umm al-Qura date from 1990 was produced by a different rule from one printed in 2026. Van Gent's Umm al-Qura date converter covers 1356 AH (14 March 1937) to 1500 AH (2077), and notes that its dates for 1356 to 1411 AH come from a conversion table published by the King Fahd University of Petroleum and Minerals in the early 1990s. Different converters handle these older years in different ways, which is one reason Hijri date converters can disagree by a day.

A civil calendar, not a moon-sighting announcement

Van Gent's page on adjustments to the Umm al-Qura calendar states that "strictly speaking," the calendar "is intended for civil purposes only." Van Gent adds that the calendar's makers are aware the first sighting of the crescent can come up to two days after the date it predicts.

For the months that carry major acts of worship, Saudi Arabia relies on sighting reports. The same page says official hilal sighting committees have operated since 1419 AH (1998/99). Before these months, Saudi Arabia's Supreme Court asks people to look for the crescent on the evening of the 29th. In March 2026, for example, Arab News reported the court urging Muslims across the Kingdom to look for the Shawwal crescent on the evening of 29 Ramadan 1447 and to report any sighting to the nearest court.

So the start of Muharram, Ramadan, Shawwal and Dhu al-Hijjah can be moved from the printed date. Van Gent's comparison table on the adjustments page, covering 1422 AH onwards, shows:

• Ten cases where the announced month began a day after the Umm al-Qura date, including Ramadan 1424, Muharram 1435, Ramadan 1439 and Muharram 1443.
• Seven cases between 1425 and 1429 AH where the announced month began a day before the Umm al-Qura date, on evenings when the moon set just before or even well before sunset. Van Gent, citing published studies, regards such early reports as mistaken sightings.
• In one case, the change produced a 31-day month: both 28 and 29 December 2007 were counted as 19 Dhu al-Hijjah 1428.

In the same table, the announced dates for these four months match the calculated ones from Ramadan 1443 (April 2022) through Muharram 1448 (June 2026). That record says nothing certain about the next announcement, which still depends on the sighting reports of that evening.

Outside Saudi Arabia

Outside Saudi Arabia, the gap can be larger. Van Gent notes that the dates observed by different Muslim communities around Ramadan and Dhu al-Hijjah can differ by one, two or even three days. Many communities start each month by local sighting, which the US Naval Observatory describes as the traditional practice: the month begins when the thin crescent is actually seen after sunset. Others follow a calculation or another country's announcement. Which method to follow is a religious question for qualified scholars and your local mosque; moon sighting vs calculation sets out the main positions.

Where you will meet Umm al-Qura dates

• Saudi government use. The government uses the calculated calendar for civil purposes, so Hijri dates on official paperwork normally follow it. Reading Hijri dates on Saudi documents explains how to convert them.
• Software and websites. Microsoft's .NET includes it as the UmAlQuraCalendar class, and many apps offer it as a Hijri option. The Hijri Calendar site uses it too: every date in its converter and monthly calendars is a calculated Umm al-Qura date.
• Planning ahead. It is a common basis for printed calendars and for estimating when Ramadan or Eid will fall. Those estimates stay provisional until the announcement you follow.

One detail catches people out when converting. A Hijri date starts at sunset, while a Gregorian date starts at midnight. Converters, van Gent's included, give the Gregorian day that follows that sunset, so the Islamic date has already begun on the previous evening.

Common questions

Is the Umm al-Qura calendar the same as the tabular Islamic calendar?

No. The tabular calendar uses a fixed arithmetic pattern: months alternate between 30 and 29 days, with 11 leap days in a 30-year cycle, as the US Naval Observatory explains. The Umm al-Qura calendar instead tests the real positions of the sun and moon at Makkah for every month, so its 29- and 30-day months follow no fixed pattern. Because the two follow different methods, the same day can carry different Hijri dates in each. The tabular Islamic calendar explains the arithmetic version.

Does the Umm al-Qura calendar decide the date of Hajj?

Not on its own. Dhu al-Hijjah is one of the months whose start Saudi Arabia confirms through sighting reports, and the day of Arafah and Eid al-Adha follow that announced start. The calculated date is a forecast that is usually, but not always, confirmed.

Using the calendar in practice

For civil dates, Saudi paperwork and long-range planning, the calculated Umm al-Qura date is the standard reference. For Ramadan, Eid and Hajj, treat it as a planning estimate and confirm the start of the month with the authority you follow.

To work with it:

• Convert any date with the Hijri date converter, keeping in mind that dates from before 1423 AH followed older rules.
• Check the current calculated date on the Hijri date in Saudi Arabia page.
• Look ahead to the next month in the Jumada al-Thani 1448 calendar, or start from the calendar home page.