The source: the Umm al-Qura calendar
We use the Umm al-Qura calendar, the official Islamic calendar of Saudi Arabia. It is calculated in advance: a month begins on the day after the 29th if, seen from Makkah, the new moon (conjunction) happens before sunset on the 29th and the moon sets after the sun; otherwise the month runs to 30 days. That is why every month’s length, 29 or 30 days, is known before it begins.
The date engine
Dates are converted with the Umm al-Qura table built into the ICU calendar library that browsers and operating systems use, limited to 1343–1500 AH (from about 2 August 1924 to November 2077). The site rejects any date outside that range or that does not exist, such as the 30th of a 29-day month, and says why.
How we check it
We copied the start date and length of every month of 1440–1450 AH from the official Umm al-Qura calendar website: 132 months, 70 of them 30 days long. Our tests check the engine against each one: the day it starts, its length, its last day and the change to the next month. These tests run with the tool tests before every update, and nothing is published if one fails.
Moon sighting
The start of Ramadan, Shawwal and Dhu al-Hijjah is announced officially after the Supreme Court calls for the crescent to be sought on the evening of the 29th of the previous month. The announced date can differ from Umm al-Qura by a day, so we describe Ramadan, the Eids and the Day of Arafah as expected dates and give the sighting evening on each page.
When an announcement differs from the calendar, we record it on the site: the pages for that occasion then show the announced date and count down to it, while the calendar itself stays on the Umm al-Qura table.
Time and the start of the day
All pages use Riyadh time (UTC+3). On calendars and in official use the date changes at midnight; in religious terms the Islamic day begins at sunset, and we point this out where it matters. Pages are rebuilt every night after midnight, and a page saved from an earlier day updates its date in your browser.
Other calculations
- Ages and durations: whole years, then whole months, then the remaining days, in each calendar with its own month lengths. If the same day doesn’t exist in a month, that month is complete on its last day.
- Working days: Sunday to Thursday; public holidays are not taken out.
- Sunset, dawn and moon phases: astronomical calculations for Riyadh with the astronomy-engine library, with dawn when the sun is 18.5° below the horizon. They are estimates; for prayer times follow the official Umm al-Qura timetable or your mosque.
Updates and corrections
We review the content and calculations against the official table regularly, and the date of the last review appears at the bottom of each page. If you find a mistake, write to us from the contact page with the page link and the date in question; we will check it, correct it and update the review date.