Quick Answer
- Gregorian: a solar calendar, 365 or 366 days, kept in step with the seasons
- Hijri: a lunar calendar, 354 or 355 days, kept in step with the moon's phases
- The gap: about eleven days a year, which the Hijri calendar deliberately never corrects
- The effect: the same Islamic date moves earlier every year and passes through every season in about 33 solar years
- Conversion: no single formula works for a date, because Hijri month lengths vary; a table or a fixed calendar is needed
Two calendars run in the same house. One decides when the school term starts; the other decides when the fast begins.
The difference between them is not a matter of accuracy. It is a difference of purpose, and once you see what each is designed to track, the arithmetic stops looking strange.
What Is the Actual Difference Between Them?
One follows the sun and the other follows the moon, and that single choice decides everything else.
The Gregorian year is built to stay in step with the seasons: it counts 365 days and adds a day in leap years to keep the average close to the 365.2422-day tropical year. The Hijri year is built on twelve lunar months, each beginning with the crescent, which gives 354 or 355 days and no correction back to the seasons at all.
The result is that a Gregorian date tells you what the weather will be like. A Hijri date tells you where the moon is. Neither is doing the other's job.
How Does the Gregorian Calendar Stay in Its Season?
By adjusting the leap years with unusual care.
The calendar in use today went into effect in October 1582, introduced by Pope Gregory XIII as a correction of the Julian calendar. The change looked small and was decisive: instead of a leap year every four years without exception, the new rule makes every year divisible by four a leap year, except years divisible by 100 that are not also divisible by 400. So 1800 and 1900 were not leap years, while 1600 and 2000 were.
That rule brings the average year to 365.2425 days, against a tropical year of about 365.2422. The error is now a fraction of a day per century instead of nearly a day, and the seasons hold their places in the calendar.
The reason this matters to the comparison is that it shows what a solar calendar is for. Every correction in the Gregorian system exists to stop the date from drifting away from the sun.
How Does the Hijri Calendar Keep Time?
By following the moon and refusing to correct the drift.
The Hijri year has twelve lunar months and 354 or 355 days. Its epoch is the Hijrah of 622 CE, and its years are written AH. A month begins with the sighting of the crescent after sunset, which is why its length is 29 or 30 days rather than a fixed number, as the separate explainer on why Islamic dates depend on moon sighting sets out.
That produces two versions of the same calendar in practice. One follows the sighting and is announced month by month. The other is calculated in advance: Saudi Arabia has for decades used a calculated lunar calendar called Umm al-Qura, which is followed for civil purposes in several Gulf states and is the default Islamic calendar in the Arabic settings of Microsoft software. Communities that follow the sighting and communities that follow the calculated calendar can therefore be a day apart, and both are following a recognised method.
Why Do Eleven Days Add Up to a Whole Year?
Because the shortfall is small each year and has nothing to correct it.
The average time between one new moon and the next is about 29.53 days. Twelve of those give a year of roughly 354.37 days, against 365.24 for the solar year: a gap of about 10.9 days, which is the eleven days people notice each year when Ramadan arrives earlier than last time.
Add the gap up and it compounds. Thirty-three Hijri years come to about 32 solar years, which is why the astronomical literature describes the twelve lunar months as regressing through the seasons over a period of about 33 solar years. In practice that means every Islamic date eventually falls in every season: a fast observed in winter in one decade will be observed in summer two decades later.
A solar calendar would treat that as a failure and insert a leap month to stop it. The Hijri calendar does not, and a large part of the tradition regards the absence of any such insertion as the point rather than an oversight.
Side by Side
| Property | Gregorian | Hijri |
|---|---|---|
| Basis | The sun: a tropical year of about 365.2422 days | The moon: a synodic month of about 29.53 days |
| Months | 12, with lengths fixed by rule | 12, each of 29 or 30 days |
| Year length | 365 or 366 days | 354 or 355 days |
| Correction | Leap years: every fourth year, except centuries not divisible by 400 | No leap month; calculated versions use eleven leap days across a thirty-year cycle |
| Epoch | Continuous CE numbering | AH, counted from the Hijrah in 622 CE |
| Relation to seasons | Fixed: the same month means the same season | Drifts: the whole year moves through the seasons in about 33 solar years |
| Main use | Civil life, schools, offices, contracts | Religious dates, fasting, pilgrimage, mourning calendars |
How Do You Convert a Date From One to the Other?
For a full date, you do not, not with arithmetic alone.
A conversion needs to know how long each Hijri month is, and that is not fixed by a formula: it depends on the sighting or on the calculated calendar being used. That is why date conversion is done with a table, an app or an official calendar rather than a mental sum. The year number is easier. Because a Hijri year is about eleven days shorter, the Hijri count runs ahead of the Gregorian count by roughly a year every 32 to 33 solar years, which is enough to place an event in the right century and not enough to name the right day.
The calculated calendars solve this by fixing month lengths in advance. Umm al-Qura is the most widely used of them, followed for civil purposes in Saudi Arabia and several neighbouring states and named by several Muslim bodies abroad, among them the Fiqh Council of North America and the European Council for Fatwa and Research, as the calendar they follow. It is also the source of the Islamic dates shown by a great deal of software.
It remains contested. Communities that base the calendar on the actual sighting of the crescent criticise the calculated approach precisely because it produces a tidy date instead of an observed one. In practice this means a converted date should be treated as a prediction: the number is right about the year, and the day can move by one.
The Next Four Years, Calculated
The table below gives the calculated dates for the main dates on the mourning and worship calendar, with the expected South Asian date one day later, which is the common outcome where the month follows a local sighting rather than a calculated calendar.
| Hijri year | 1 Muharram | Ashura, 10 Muharram | 1 Ramadan | Eid al-Fitr, 1 Shawwal | Eid al-Adha, 10 Dhu al-Hijjah |
|---|---|---|---|---|---|
| 1449 | 6 Jun 2027 7 Jun 2027 | 15 Jun 2027 16 Jun 2027 | 28 Jan 2028 29 Jan 2028 | 26 Feb 2028 27 Feb 2028 | 5 May 2028 6 May 2028 |
| 1450 | 25 May 2028 26 May 2028 | 3 Jun 2028 4 Jun 2028 | 16 Jan 2029 17 Jan 2029 | 14 Feb 2029 15 Feb 2029 | 24 Apr 2029 25 Apr 2029 |
| 1451 | 14 May 2029 15 May 2029 | 23 May 2029 24 May 2029 | 5 Jan 2030 6 Jan 2030 | 4 Feb 2030 5 Feb 2030 | 13 Apr 2030 14 Apr 2030 |
| 1452 | 3 May 2030 4 May 2030 | 12 May 2030 13 May 2030 | 26 Dec 2030 27 Dec 2030 | 24 Jan 2031 25 Jan 2031 | 2 Apr 2031 3 Apr 2031 |
Two warnings belong with that table. The top line in each cell is a calculated figure, not an announcement, and the second line is an expectation rather than a ruling; where the crescent is not sighted, a month is completed at thirty days and the dates after it move again. Use the table for planning, and the local announcement for practice.
Why Living With Two Calendars Is Not a Contradiction
Because they are tracking different things, and a community needs both.
The Gregorian calendar is the working calendar of offices, schools, contracts and travel, and no religious authority disputes its use for those purposes. The Hijri calendar is the calendar of worship, and its dates are what decide when the fast begins, when the pilgrimage is performed and when the first ten days of Muharram are observed.
The drift that looks like a defect from the solar side is the feature from the religious side. A calendar tied to the moon puts every month of the year in reach of every season, so a child who grows up fasting will see the fast in winter and in summer if they live long enough. That is a form of fairness the solar year cannot offer.
For the mechanics of the lunar year itself, see the Hijri year, month by month. For the season that matters most on this site, see the meaning of Muharram and what Ashura is.
Key Takeaways
- The Gregorian calendar follows the sun and corrects itself with a careful leap-year rule; the Hijri calendar follows the moon and does not correct at all.
- A Hijri year is 354 or 355 days, about eleven days shorter than a solar year.
- Because there is no leap month, Islamic dates move earlier each year and cross every season in about 33 solar years.
- A full date cannot be converted by arithmetic alone, since Hijri month lengths vary; calculated calendars such as Umm al-Qura solve this by fixing them in advance.
- The calculated tables are for planning. The announced date is what the community acts on.
Frequently Asked Questions
What is the main difference between the Hijri and Gregorian calendars?
The basis. The Gregorian year follows the sun and stays locked to the seasons at 365 or 366 days. The Hijri year follows twelve lunar months at 354 or 355 days and drifts through the seasons by design.
Why are Islamic dates about eleven days earlier every year?
Because twelve lunar months come to roughly 354.37 days against 365.24 for the solar year, a shortfall of about 10.9 days. Nothing in the calendar corrects it, so the gap is carried forward each year.
How do you convert a Hijri date to a Gregorian date?
With a table, an app or an official calendar rather than mental arithmetic, because the length of each Hijri month depends on sighting or on a calculated scheme. A converted date should be treated as a prediction that can be a day out.
What is the Umm al-Qura calendar?
A calculated lunar calendar used in Saudi Arabia for several decades, followed for civil purposes in several Gulf states and adopted by some Muslim bodies abroad. Communities that follow actual sighting often criticise it for that reason.
Does the Hijri calendar have leap years?
No leap month and no correction back to the seasons. Calculated versions spread eleven leap days across a thirty-year cycle, but that is arithmetic inside a lunar scheme, not an extra month.
Which calendar decides when Ashura is observed?
The Hijri one. Ashura is the tenth of Muharram, so it follows the announcement that opens Muharram, which is why its Gregorian date changes every year.
Sources
- Wikipedia, "Gregorian calendar", on the calendar going into effect in October 1582 under Pope Gregory XIII as a replacement for the Julian calendar, the average year of 365.2425 days against a tropical year of about 365.2422, and the leap-year rule with its century exception, at Wikipedia.org.
- Wikipedia, "Islamic calendar", on the twelve lunar months and the 354 or 355 day year, the Hijri era from 622 CE, the mean synodic month of 29.5306 days, the 10.87 day shortfall, the regression of the lunar year through the seasons over about 33 solar years, the tabular calendar, and the Umm al-Qura calendar as a calculated scheme used for administrative purposes, at Wikipedia.org.
- R. H. van Gent, "The Umm al-Qura Calendar of Saudi Arabia", on Saudi Arabia's calculated lunar calendar, its adoption in Bahrain, Qatar and the UAE, its use in software and by the Fiqh Council of North America and the European Council for Fatwa and Research, and the criticism from communities that base the calendar on sighting, at Utrecht University.
- Wikipedia, "Ashura", for Ashura falling on the tenth of Muharram, at Wikipedia.org.
Disclaimer: The calculated dates in the tables are for planning and are generated from the Umm al-Qura calendar, which is a calculation and not an announcement. The second date in each cell is an expectation for South Asian communities that follow local sighting, not a ruling for any particular country. For practice, follow the announcement of your local community. If you find an error, write to us through the contact page and we will correct it.
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