What Is the Hijri Calendar? Its Origin, Structure, and Difference from the Gregorian Calendar

What is the Hijri calendar?

The Hijri calendar is a purely lunar calendar: its months are defined by the Moon’s cycle around the Earth, not by the seasons. A Hijri month begins with the birth of the new crescent and lasts 29 or 30 days; the year has twelve months — 354 or 355 days — about 11 days shorter than the (solar) Gregorian year. Its epoch is the Prophet Muhammad’s migration (Hijra) from Makkah to Madinah in 622 CE, which is why its years are called “Hijri years” and marked “AH”.

Because the Hijri year is shorter than the solar one, Islamic occasions — Ramadan, Hajj — advance by about 11 days in the Gregorian calendar each year, completing a full cycle through the four seasons roughly every 33 years: a Ramadan that falls in mid-summer heat this decade will fall in mid-winter three decades later.

From the pre-Islamic “nasi” to a purely lunar calendar

Pre-Islamic Arabs did not follow a purely lunar calendar; they kept their lunar months aligned with the seasons by intercalating a thirteenth month called nasīʾ every few years, making their calendar lunisolar like the Hebrew one. The Quran abolished the practice: “Intercalation is but an addition to disbelief” (9:37), and in the Farewell Pilgrimage the Prophet ﷺ said: “Time has completed its cycle as it was the day Allah created the heavens and the earth; the year is twelve months, four of them sacred” (Sahih al-Bukhari 3197). From then on the Islamic calendar became purely lunar, with no tie to the seasons — the direct cause of its months drifting through them.

When and how was Hijri dating adopted?

The Hijra occurred in 622 CE, but dating by it was not adopted during the Prophet’s lifetime; documents were dated by major events instead. Historical sources — al-Tabari’s history among them — relate that Hijri dating was settled in the caliphate of Umar ibn al-Khattab: his governor in Basra, Abu Musa al-Ashari, wrote that undated correspondence reached him and he could not tell which came first. Umar consulted the senior Companions, and Ali ibn Abi Talib proposed dating from the migration of the Messenger of Allah ﷺ. The proposal was adopted, and Muharram — not the month of the actual migration (Rabi al-Awwal) — was chosen as the year’s first month; 1 Muharram 1 AH thus corresponds to mid-July 622 CE.

One consequence is worth noting: the first Hijri year begins about two months before the Prophet ﷺ actually arrived in Madinah, because Muharram precedes the Rabi al-Awwal of the migration. Reports on the year of adoption vary between 16 and 18 AH, with about 17 AH (638 CE) being the most cited; a recent peer-reviewed study concluded that competing Arabian calendars remained in use until about 17 AH, after which the Hijri calendar became the definitive calendar of the Muslims.

The structure of the Hijri year

The Hijri year has twelve months: Muharram, Safar, Rabi al-Awwal, Rabi al-Akhir, Jumada al-Ula, Jumada al-Akhira, Rajab, Shaban, Ramadan, Shawwal, Dhul-Qada, and Dhul-Hijjah. Four are sacred: Dhul-Qada, Dhul-Hijjah and Muharram consecutively, and Rajab alone. A month has 29 or 30 days set by sighting or computation, and the average Hijri year is 354.37 days. See the complete table of months with the meanings of their names in Months of the Hijri Year, and month-by-month calendars for 1448 AH on the Hijri year page.

Hijri vs Gregorian: what is the difference?

The essential difference is each calendar’s reference: the Gregorian is solar and keeps the seasons fixed (the equinoxes and solstices barely move in it), while the Hijri is lunar and keeps the moon phases fixed. The consequences:

• The Hijri year is 354–355 days against 365–366 Gregorian — about 11 days shorter each year.
• The two years even out roughly every 33 years: 100 Gregorian years ≈ 103 Hijri years.
• Age in Hijri years runs ahead of Gregorian age: someone turning 40 by the Gregorian count has completed about 41 Hijri years — exactly what the Hijri age calculator computes.

The Hijri calendar in Muslim life today

The Hijri calendar is the official calendar of Saudi Arabia (through the Umm al-Qura calendar, detailed in The Umm al-Qura Calendar) and is used alongside the Gregorian in official documents in several countries. All timed acts of worship rest on it: the Ramadan fast, Eid al-Fitr dues, Hajj, Ashura and Arafah. It is also the default Islamic calendar in Windows systems set to Arabic. To see today’s Hijri date instantly or convert between the two calendars, use today’s date page or the date converter on this site.

Sources and references

• Umm al-Qura University, Makkah — the official reference for the Umm al-Qura calendar adopted in Saudi Arabia.
• Encyclopaedia Iranica — Calendars in the Islamic Period (Reza Abdollahy): a peer-reviewed study of the adoption of Hijri dating under Caliph Umar.
• Zein & El-Wakil, Religions 12(1):42 (2021): a peer-reviewed study of Hijri calendar origins, the pre-Islamic nasiʾ, and the competing calendars until 17 AH.