You check the Hijri date on your phone, then see a different one on a mosque noticeboard, an Islamic app, or a relative's phone. It is a small thing, but it is disconcerting: a date feels like it ought to be a fact.
It is not that one of them is broken. There are several Hijri calendars, your phone ships more than one of them, and apps pick different ones. We measured how far apart they actually get.
Your phone ships four Hijri calendars
Modern operating systems and browsers implement several Islamic calendar variants side by side. The four in common use are the Umm al-Qura calendar used officially in Saudi Arabia, a calculated astronomical variant, the tabular civil calendar, and a tabular variant using a different epoch. An app picks one, often without telling you which.
On the day this was written, the four gave three different answers for the same instant:
- Umm al-Qura. Rabi' I 29, 1448
- Astronomical. Rabi' I 30, 1448
- Civil (tabular). Rabi' I 28, 1448
- Tabular, alternate epoch. Rabi' I 29, 1448
How often they disagree: every single day
We ran all four across every day of 2026 and compared the dates they produce. The result is more extreme than we expected.
On **none** of the 365 days do all four calendars agree. On 216 days they produce two distinct dates, and on 149 days they produce three or more — with days on which all four differ from one another. Umm al-Qura and the civil calendar alone disagree on 278 days of the year, once for a run of 150 consecutive days.
So a Hijri date shown without saying which calendar produced it is close to meaningless as a precise claim. It is roughly right — the month and the rough position within it — and that is all.
Why the variants exist at all
The tabular calendars are arithmetic: months alternate 30 and 29 days with a fixed leap pattern, so the date can be computed centuries ahead and never needs an observation. That is what makes them useful in software and for administrative dating, and also what guarantees they drift from what anyone actually sees in the sky.
Umm al-Qura is more sophisticated: it is calculated, but using criteria based on the moon's position over Mecca, and it is the official civil calendar of Saudi Arabia. It is close to observation but is not observation, and it is specific to one location's horizon.
The astronomical variant tries to model actual visibility. It gets closest to a sighting-based month and is correspondingly the least suitable for printing a calendar years in advance.
None of them is the Hijri calendar. As the Ramadan dates post sets out, the month traditionally begins with a sighting, and no algorithm reproduces that exactly.
What this means in practice
For everyday use — knowing roughly where you are in the month, or that Rajab has begun — any of them is fine, and the difference of a day is not worth worrying about.
It matters for two things. First, for the start of Ramadan and for the two Eids, where a day's difference is the whole question and the answer comes from your local authority, not your phone. Second, for anything you are counting: fasting the white days, or working out when a fixed number of days has elapsed, where switching between apps mid-count will quietly corrupt the tally.
The honest thing for software to do is say which calendar it uses. This site uses Umm al-Qura for the Islamic calendar and the date shown alongside prayer times, because it is the most widely recognised civil reckoning — and, as everything above should make clear, it is a convention we picked, not the truth of the matter.
Common questions
- Why is the Hijri date on my phone different from the mosque's?
- Because they are almost certainly using different Islamic calendars. Phones ship several — Umm al-Qura, astronomical, and two tabular variants — and across 2026 there is not a single day on which all four produce the same date.
- Which Hijri calendar is the correct one?
- None of them is the calendar in the traditional sense, because the month is traditionally begun by sighting the crescent rather than by calculation. Umm al-Qura is the most widely used civil reckoning; the tabular calendars are arithmetic conveniences that drift from observation by design.
- How far apart do the Hijri calendars get?
- Measured across 2026: two distinct dates on 216 days, three or more on 149 days, and Umm al-Qura versus the civil calendar disagreeing on 278 days including one run of 150 consecutive days.
- Does this affect Ramadan and Eid dates?
- It is exactly where it matters. For the start of Ramadan and for both Eids, follow your local mosque or national authority rather than a calculated date on a phone.