Methodology
How we calculate Hijri dates
There is no single authoritative Hijri date. That sentence surprises people, but it is the honest starting point — and everything on this site follows from it. There are three legitimate ways to know today's Islamic date, and they do not always agree.
The three ways to know the Hijri date
1. Umm al-Qura — our default
Saudi Arabia's official calendar. It is a calculation, but a careful one: each month's length is set by predicted moon visibility over Mecca, using astronomical criteria rather than a fixed cycle. It is the calendar behind Saudi civil dates and the one most Hijri date websites show, so it is what you see first here — computed in your browser with the platform-native Islamic calendar, no tables shipped, no API calls. We cross-check it against an independent copy of the Umm al-Qura tables (details below).
2. The tabular (civil) calendar — the toggle
This is pure arithmetic: a 30-year cycle in which 11 years are leap years, months alternate between 29 and 30 days, and the whole thing can be computed forever in both directions. It is the same everywhere on earth. Our implementation follows the published Reingold–Dershowitz arithmetic, verified against ICU's independent islamic-civil calendar. It never needs the internet, never breaks, and never surprises you.
Its weakness is also its strength: because it is arithmetic, it cannot know when a crescent was actually seen. It often runs a day — occasionally two — behind Umm al-Qura and the observed calendar.
Umm al-Qura and the tabular calendar agree on most months and differ by a day on some. When they differ, neither is wrong; they are answering slightly different questions.
3. Local moon sighting — the final word
The actual practice: on the 29th evening of each Islamic month, committees and volunteers look for the new crescent. Seen, the new month begins; not seen, the current month completes thirty days. Because the crescent becomes visible on different evenings in different places — and because some countries require a local sighting while others follow Saudi Arabia — two countries can begin Ramadan on different days and both be completely correct.
This is why every tool and countdown on this site carries the same line: the calculated date is our best answer, and your country's moon-sighting announcement has the final say. For Ramadan, the two Eids and Hajj, always follow the local announcement.
The HIJRI_ADJUSTMENT setting
Major Hijri services let you nudge all dates by a day or two to match local sighting. We built the same control in: a single global setting, currently set to 0 (pure calculation), applied to every date on the site. If moon sighting in your region ever runs consistently ahead of or behind the calculation, that one number is how the whole site follows it.
How we check our work
Trust is the product here, so we verify the math rather than asserting it. For the 1st of every month in Hijri years 1447 and 1448 (24 dates), we check:
- Our tabular implementation against the ICU islamic-civil calendar — an independent implementation of the same arithmetic. Result: 24 of 24 exact matches.
- Our Umm al-Qura source (platform-native Intl) against moment-hijri's embedded Umm al-Qura table — an independent copy of the Saudi data. Result: 24 of 24 exact matches.
- Tabular against Umm al-Qura: they agree exactly or within a day on 22 of 24 months, and differ by two days on two months (Safar and Rabi' al-Thani 1448) — genuine divergence between the two calendars when Umm al-Qura strings two 29-day months together, confirmed by a third Umm al-Qura implementation. This is exactly the case the method toggle exists for.
The full 24-date log ships in the project's README, and the check re-runs automatically with every build.
What this means for you
For everyday questions — converting a birthday, checking today's date, planning around a month — the calculated date is all you need. For the dates that shape a community's worship, the calculation gets you to the right neighborhood and the moon takes you home. We would rather show you both methods and the honest caveat than one confident number.
