If you have ever wondered why Ramadan lands on a different date each year, or why your Islamic birthday never matches your regular one, you have run into the basic difference between the Hijri and Gregorian calendars. One follows the moon. The other follows the sun. That single choice explains almost everything else about how they behave.
The Basic Idea Behind Each Calendar
The Gregorian calendar is the one most of the world uses for daily life, business and travel. It was introduced by Pope Gregory XIII in 1582 as a correction to the older Julian calendar, which had slowly drifted out of step with the seasons. It is a solar calendar, which means it is built around the time the Earth takes to orbit the sun, roughly 365.24 days.
The Hijri calendar, also called the Islamic calendar, is a lunar calendar. Each month begins with the sighting of the new crescent moon, and a month lasts either 29 or 30 days depending on the moon’s cycle. Its starting point is the Hijrah, the migration of the Prophet Muhammad from Makkah to Madinah in 622 CE. Years are counted from that event and marked AH, short for “Anno Hegirae.” The Gregorian calendar uses CE, or AD in older usage, counting from a different reference point.
A Side-by-Side Comparison
The table below puts the main differences in one place.
| Feature | Hijri Calendar | Gregorian Calendar |
|---|---|---|
| Based on | Cycles of the moon | Earth’s orbit around the sun |
| Length of year | 354 or 355 days | 365 or 366 days |
| Months per year | 12 | 12 |
| Length of month | 29 or 30 days | 28, 29, 30 or 31 days |
| Starting point | Hijrah in 622 CE | Traditional birth year of Jesus |
| Day begins at | Sunset | Midnight |
| Extra day rule | One day added to the last month in some years | One day added to February in leap years |
| Seasons | Move through all seasons over time | Stay fixed in the same months |
| Month start decided by | Moon sighting or astronomical calculation | Fixed rules, no observation needed |
| Main use | Religious observance, some civil use | Global civil and commercial use |
| Year count in late 2026 | About 1448 AH | 2026 CE |
Both calendars have twelve months and seven-day weeks, so they can look similar on the surface. The differences sit underneath, in how the months and years are actually measured.
Why the Hijri Year Is Shorter
A lunar month, meaning the time from one new moon to the next, averages about 29.53 days. Since a calendar month has to be a whole number of days, the Hijri months alternate between 29 and 30 days. Multiply that average by twelve and you get roughly 354.37 days. That is the length of a Hijri year.
The Gregorian year averages 365.2425 days, because it adds a leap day in most years divisible by four, with a few exceptions for century years. So the gap between the two comes down to simple subtraction: about 365.24 minus 354.37, which leaves close to 10.9 days.
That is where the familiar figure comes from. In a normal Hijri year of 354 days the gap is around 11 days, and in a Hijri leap year of 355 days it is about 10. Across the long run it averages out to just under 11, which is why people say “10 or 11 days” and mean both.
How the Shift Shows Up in Real Life
The clearest example is Ramadan. In 2024 it began around March 11. In 2025 it started on about March 1, which is 10 days earlier. In 2026 it began around February 18, and in 2027 it is expected close to February 8. Each year the month slides back through the Gregorian calendar by a little under two weeks.
Because of this, no Islamic holiday stays in one season. Ramadan has fallen in the heat of summer and in the middle of winter, and it will do both again. The same goes for Eid al-Fitr, Eid al-Adha, and the start of the Hijri new year. Muslims fasting in the long, hot days of a northern summer will, about 15 years later, be fasting through short winter days, and the reverse for those in the southern hemisphere. Over time everyone gets a turn at both.
The 33-Year Cycle
Since the Hijri year is about 11 days shorter, it takes a while for it to lap the Gregorian calendar completely. The rough answer is 32 to 33 years. After around 32.6 solar years, the Islamic calendar has gained one full extra year on the Gregorian one, and a given Hijri date returns to the same season.
This is why there is a handy rule of thumb for converting years. Thirty-three Hijri years are about equal to thirty-two Gregorian years. If you want to estimate the Hijri year from a Gregorian one, subtract 622, multiply by 33, and divide by 32. For 2026 that works out to about 1448, which matches the calendar in use today. It is only an approximation, but it is good enough to keep you in the right year.
How Each Calendar Handles Corrections
Every calendar has to deal with the fact that nature does not divide neatly into whole days. The Gregorian calendar fixes this with leap years. It adds February 29 in years divisible by four, skips the extra day in century years unless they are divisible by 400, and so keeps the seasons anchored to the same months. That is why the spring equinox falls around March 20 or 21 nearly every year.
The Hijri calendar has a different problem, since its months are supposed to follow the actual moon. In the traditional system, the start of each month depends on whether the crescent is sighted. In the tabular version used for long-range planning, 11 years in every 30-year cycle receive an extra day at the end of the final month, Dhu al-Hijjah. That brings the average close to the true lunar cycle and keeps the tables from drifting.
Moon Sighting and Calculation
There is one more practical difference worth knowing. A Gregorian date can be worked out years in advance without looking at the sky. A Hijri date, in the traditional method, depends on observation. Communities watch for the new crescent at dusk on the 29th day of the month. If it is seen, the next day starts the new month. If clouds or distance make it impossible, the month runs to 30 days.
This is why two countries can sometimes begin Ramadan or Eid on different days. Some follow local sightings, some follow sightings anywhere in the Muslim world, and others rely on astronomical calculation. Saudi Arabia uses the Umm al-Qura calendar, which is based on calculation rather than direct sighting, and it is widely used for administrative purposes in the Kingdom. A single day’s difference between regions is normal and does not mean anyone has made a mistake.
The Start of the Day
The two calendars also disagree about when a day begins. In the Gregorian system, a new day starts at midnight. In the Hijri system, it starts at sunset. That means the night before a date belongs to that date. For example, the first night of Ramadan is counted as part of the first day, which is why the Taraweeh prayers are held the evening before the first fast.
Which One Should You Use?
For most practical purposes, people use both. The Gregorian calendar handles work, school, flights and contracts, since it is the shared standard almost everywhere. The Hijri calendar handles Ramadan, Hajj, Eid, the sacred months and other religious dates. Many phones and planners now show both side by side, which makes the yearly shift easy to see.
The main thing to remember is that neither calendar is wrong or better. They simply measure time against two different clocks in the sky. The sun gives a steady seasonal year, and the moon gives a shorter one that walks through the seasons. Once you see that the gap is only the difference between 12 moon cycles and one trip around the sun, the 10 to 11 day shift stops feeling mysterious and starts to feel like simple arithmetic.