Converting Hijri to Gregorian: the complete guide, and why results disagree

Open three date-conversion sites at once, enter the same date, and you may well get three answers — two that agree and one that sits a full day earlier or later. That is not a passing bug on one of them. It is the predictable result of the three sites using different methods, and understanding the difference is what separates a date you can put on an official form from a date that is merely close.

The short answer

The Hijri calendar used officially in Saudi Arabia is the Umm al-Qura calendar. Its month lengths are computed astronomically for the coordinates of Mecca and published in advance as official tables. Any converter that does not use those tables — one that relies on an arithmetic formula, or on the calendar built into your browser — will disagree with the official date on a large share of days, and especially on older dates. Sahel's converter uses the published table itself, and shows today in both calendars the moment you open it.

Why does the Hijri date differ between websites?

Because a developer has three ways to produce a Hijri date, and they do not agree:

1. The arithmetic (tabular) calendar: assumes months alternate 30 and 29 days on a fixed pattern, with leap years on a thirty-year cycle. Fast and simple, and it knows nothing about the actual moon.
2. The calendar built into the browser or operating system: in practice whatever the ICU library provides, which is what browsers and most programming languages lean on. Better than the first, but still not the official table.
3. The published Umm al-Qura table: the real month lengths as adopted and published, year by year.

The gap between the second and the third is wider than most people expect. Compared across the full range from 1343 AH to 1500 AH — close to fifty-six thousand days — the two disagree on roughly 39% of days. They agree only within a narrow window running from about 1420 AH to 1450 AH, the years around our own time. Before that, divergence is the rule rather than the exception.

A decisive example: 1 Muharram 1400 AH falls on 20 November 1979 in the official published calendar. Approximate methods place it on 19 November, a full day earlier.

This is why the "my date is off by a day" complaint clusters so heavily around older birth dates: most births before 1999 fall in the region where approximate methods drift.

What exactly is the Umm al-Qura calendar?

Umm al-Qura is the official state calendar of Saudi Arabia, and it is the basis for dates in documents, contracts and government transactions. It is not built on a fixed formula but on an astronomical calculation of lunar visibility from the coordinates of Mecca, which is then adopted and published in tables ahead of the year.

Because it rests on astronomical observation rather than a mathematical pattern, its behaviour is not perfectly regular. In the published historical record for the early years — specifically between 1343 AH and 1364 AH — eight months break the pattern entirely, some running 28 days and some 31. No arithmetic scheme, however refined, can produce those. They are not typographical errors; they are what was actually adopted at the time.

The official calendar and moon sighting are not the same thing

This is where many people conflate two things. Umm al-Qura is an administrative calendar, published ahead of time so that work, appointments and payroll can be organised. The start of months with religious weight — the beginning of Ramadan, Eid al-Fitr, the Day of Arafah — is determined by actual sighting of the crescent, and can differ from the published calendar by a day.

So if your question is "what is today's Hijri date for an official form?", the answer is the Umm al-Qura calendar. If your question is "when does Ramadan begin?", the answer comes from the announcement of the competent authority, not from any converter.

Can you convert by hand?

You can reach a rough estimate. The Hijri year is about eleven days shorter than the Gregorian one, averaging 354.37 days against 365.24. That gives the familiar rule of thumb:

• Gregorian to Hijri, approximately: AH ≈ (CE − 622) × 33 ÷ 32
• Hijri to Gregorian, approximately: CE ≈ AH + 622 − (AH ÷ 33)

Note the limits, though. This gives you the year, roughly, and tells you nothing at all about the day and month. And because the Hijri year drifts through the seasons, your birthday may fall in either of two Hijri years depending on the month. Any exact day-level conversion needs the table of month lengths, and that cannot be compressed into a formula.

The Hijri months in order

The order is fixed and never changes:

1. Muharram
2. Safar
3. Rabi' al-Awwal
4. Rabi' al-Thani
5. Jumada al-Ula
6. Jumada al-Akhirah
7. Rajab
8. Sha'ban
9. Ramadan
10. Shawwal
11. Dhu al-Qa'dah
12. Dhu al-Hijjah

Each month runs either 29 or 30 days, and which one is not knowable from a fixed pattern — it is set year by year in the adopted table. So it is simply not true that "odd months have 30 days and even months 29". That is the assumption the arithmetic calendar makes, and it is exactly what produces the drift.

Why does Ramadan move earlier every year?

Because the Hijri year is about eleven days shorter than the Gregorian one, the Hijri months shift that much earlier through the Gregorian seasons each year. After roughly thirty-three years Ramadan has come full circle and returns to about the same season. This alone shows why a "fixed offset" between the calendars is a fiction: there is no constant number of years you can add or subtract to convert.

Worked examples

Example 1: an older birth date

Born on 20 November 1979. On the official table that is 1 Muharram 1400 AH — the Hijri new year exactly. A tool using the arithmetic approximation returns 30 Dhu al-Hijjah 1399 AH instead, and the person never learns their birthday was the first day of the year.

Example 2: a date outside the range

Enter a date in 1900 and you are before the start of the published table. An honest tool tells you plainly that the date is out of range; a dishonest one extrapolates a number and presents it as fact. In Sahel's converter we chose the former: out of range means a clear message, not a guess.

Example 3: the zakat year

Zakat becomes due on the completion of a Hijri year, not a Gregorian one, and an eleven-day difference per year means anyone tracking it on the Gregorian calendar quietly delays their zakat a little more each year. Convert the date you reached the nisab to Hijri first, then use the zakat calculator for the amount.

What range can you trust?

The official published table runs from 1 Muharram 1343 AH (1 August 1924) to 30 Dhu al-Hijjah 1500 AH (16 November 2077). Outside that range no adopted table exists, and any figure you see is a mathematical extrapolation with no official backing.

In practice that range covers everything a user needs: birth dates, employment and retirement dates, contracts, and future dates decades out.

Common questions

What is today's Hijri date?

The converter shows it in both calendars as soon as the page opens, with nothing to enter. The conversion runs inside your browser, so your dates are never sent to a server.

Why does the Hijri date on my ID differ from some websites?

Because official bodies use the Umm al-Qura table while many websites use the arithmetic approximation. If a site disagrees with your official document by one day, the site is usually the one that is wrong.

Is it free, and does it need an account?

Entirely free and no sign-up. It runs in the browser, so nothing leaves your device.

Try the tool

Open the Hijri and Gregorian converter to see today in both calendars, then convert in whichever direction you need. If you are working out what you are owed at work, the end-of-service gratuity calculator may help too — service periods there are counted on the Gregorian calendar, while many other financial obligations are counted on the Hijri one.