How the Islamic calendar works, and why the dates shift every year

Twelve lunar months, not twelve solar ones

The Gregorian calendar is solar. Its year is tuned to the Earth's orbit around the Sun, which is why 25 December always falls in the northern winter and why the calendar needs a leap day roughly every four years to stay there.

The Hijri calendar is purely lunar. Its year is twelve lunar months, and a lunar month - one full cycle of the Moon's phases - runs about 29.53 days. Twelve of those come to roughly 354.37 days. There is no correction, no intercalary month, and no attempt to hold the year against the seasons. That is a design decision, not an error: it is why Ramadan travels through the whole solar year over a lifetime rather than sitting in one season.

Because a month cannot have a fractional number of days, each Hijri month is either 29 or 30 days long. How that choice is made is where the different Islamic calendars part company - see the guide on Umm al-Qura, tabular and moon-sighting calendars.

The twelve months are always in the same order, always the same twelve names, and each one has its own page on this site with its Arabic spelling, its meaning and what it marks.

Where the eleven days go

A Hijri year is 354 or 355 days. A Gregorian year is 365 or 366. Subtract one from the other and you get the gap: usually 11 days, sometimes 10, and 12 when a Gregorian leap day happens to fall inside the interval.

That gap is the whole story. It is not an accumulating error and it never needs correcting. It simply means that any Hijri date - Ramadan, Eid, the Islamic New Year, your Hijri birthday - arrives about eleven days earlier in the Gregorian year than it did last time.

Here is the first of Ramadan across six consecutive Hijri years, taken from the Umm al-Qura calendar. The final column is the shift against the same date one Gregorian year earlier.

Why a month is 29 or 30 days

A lunar month is about 29.53 days, so months have to alternate between 29 and 30 days to average out. The arithmetic (tabular) Islamic calendar does this with a fixed rule: odd-numbered months get 30 days, even-numbered months get 29, and the twelfth month gains a day in a leap year.

The Umm al-Qura calendar - the official civil calendar of Saudi Arabia, and the default engine on this site - does not follow a rule. Its month lengths are published year by year in official tables, so the pattern is irregular. HijriConv embeds those tables for 1318-1499 AH, which covers roughly 1900 to 2076 in Gregorian terms.

The difference is easy to see when the two are put side by side for the same year.

Leap years without a leap day

A Hijri leap year is not a year with an extra day in February. It is simply a year of 355 days instead of 354 - the extra day is added to Dhu al-Hijjah, the twelfth month.

The tabular calendar decides this arithmetically. It runs a 30-year cycle containing 11 leap years, and a year is a leap year when (11 × year + 14) mod 30 is less than 11. That rule is exact, reproducible and needs no data at all, which is why HijriConv uses the tabular engine for years outside the Umm al-Qura table.

Umm al-Qura has no such rule. A year is long or short because the published table says so. The two systems therefore disagree about which years are leap years: 1447 AH is a 355-day year in both, but 1448 AH is a leap year in the Umm al-Qura table and a common year under the tabular rule, while 1445 AH is the reverse.

Every Hijri year page on this site states which it is, the length of each month, and the exact Gregorian day the year begins and ends.

The 33-year cycle

Since the Hijri year loses roughly eleven days against the Gregorian year, and there are roughly 365 days to lose, the two calendars come back into near-alignment after about 33 Hijri years - which is about 32 Gregorian years.

The Umm al-Qura data shows it clearly: 1 Muharram 1414 AH fell on 21 June 1993, and 1 Muharram 1447 AH, exactly 33 Hijri years later, fell on 26 June 2025. Thirty-two Gregorian years have passed and the date has crept forward by only five days.

This is the practical reason a Hijri birthday and a Gregorian birthday coincide roughly once every 33 years, and why someone's age in Hijri years is always slightly higher than their age in Gregorian years. The age calculator computes both.

It also means a single Gregorian year can contain the same Islamic observance twice, or none at all. 2030 contains two Ramadans - one beginning 5 January and the next beginning 26 December - because the Hijri year is short enough to fit twice into the same twelve Gregorian months at the edges.

What this means when you convert a date

Three practical consequences follow from everything above, and they cover almost every conversion mistake people make.

Once you know that the Hijri calendar is a lunar count with no seasonal correction, none of this is surprising - it is the calendar working as designed.

• There is no fixed offset. You cannot add a constant number of years to a Gregorian year to get the Hijri year. Any given Gregorian year overlaps two Hijri years, and the boundary moves.
• A converted date always belongs to a specific calendar. The same Hijri date can produce two different Gregorian days depending on whether you use Umm al-Qura or the tabular rule, and both answers are correct for their own system.
• A calculated date is not an announcement. Every date on this site is a civil calculation and can differ by about a day from a local crescent sighting. For fasting, Eid, Hajj and other observances, follow the announcement of your local religious authority.
• Dates far in the future are calendar arithmetic, not predictions. The Umm al-Qura table ends at 1499 AH, and beyond that HijriConv falls back to the tabular calendar and says so on the page.

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