How Umar ibn al-Khattab Adopted the Hijri Calendar

The Hijri calendar as a numbered era was started by Umar ibn al-Khattab, the second caliph, about 17 years after the Hijra. The lunar months were already in use. What Umar settled, after consulting the senior Companions, was which year would be counted as year 1. They chose the year of the Prophet's ﷺ migration from Makkah to Madinah in 622 CE, and began the count at 1 Muharram of that year. Historians date the decision to 16, 17 or 18 AH, roughly 637 to 639 CE.

Key Takeaways

• The second caliph, Umar ibn al-Khattab (ruled 634–644 CE), introduced the Hijri era. Most reports place it in 16, 17 or 18 AH (637–639 CE).
• The trigger was administrative: official letters across a growing state carried no year, so they were hard to place in order.
• Sahih al-Bukhari 3934 records that the Companions counted neither from the Prophet's ﷺ mission nor from his death, but from his arrival in Madinah.
• Year 1 starts at 1 Muharram of the Hijra year, which corresponds to 16 July 622 in the Julian calendar, not on the day of the journey.

How dates were written before Umar

Before Umar's decision there was no single, official year count. Britannica's entry on the Hijrah notes that the Prophet ﷺ himself dated his correspondence, treaties and proclamations after other events in his life.

The picture in the early sources is mixed. Reza Abdollahy's entry on calendars in the Islamic period in Encyclopaedia Iranica explains that for several years after the Hijra, some dates were reckoned by counting months from Rabi al-Awwal, the month of the migration. The historian al-Waqidi, for example, dates an expedition to Dumat al-Jandal to the 49th month after the Hijra. Other early reports give a plain year number, with years beginning in Muharram, as the old Arabian system did.

The same entry adds a caution. A few documents quoted by later historians, including letters of Khalid ibn al-Walid dated in the year 12, already seem to count from the Hijra before Umar became caliph. So the Hijra may have been used informally as a reference point earlier. What Umar did was make it the official, uniform era.

The problem: letters with no year

The pressure came from the size of the state. After the early conquests, the caliph in Madinah was corresponding with governors and commanders far away. Encyclopaedia Iranica notes that running a large territory, and collecting taxes and tribute in particular, needed one consistent calendar.

The best-known report names a governor. Paul Lunde's AramcoWorld article The Turn of a Century, written for the start of 1400 AH, recounts that Abu Musa al-Ash'ari, later governor of Kufa, wrote to Umar pointing out the difficulty: "You are sending us undated letters." A letter that names a month but no year cannot be put in order against other letters. Umar saw the risk of confusion and consulted his advisers.

The existing options did not suit. AramcoWorld's guide to the hijri and Gregorian calendars notes that Persia, Syria and Egypt each used their own epochs, and that those calendars were identified with other religions and cultures. A lunar count also had scriptural grounding. The Quran describes the moon "with precisely ordained phases, so that you may know the number of years and calculation ˹of time˺" (Surah Yunus 10:5, a translation of the meaning from The Clear Quran by Mustafa Khattab on quran.com).

The consultation and the choice of the Hijra

Umar did not decide alone. He gathered the leading Companions and put the question to them. Encyclopaedia Iranica summarises the account in the history of al-Tabari (died 923 CE): Umar asked, "from which day should we write [dates]?" Ali ibn Abi Talib answered that they should count from the day the Messenger of Allah emigrated.

The options that were set aside

Several starting points were possible, and the reports name some of them:

The hadith collection of al-Bukhari keeps the outcome in a chapter titled, on sunnah.com, "When did the Muslim calendar start?". In Sahih al-Bukhari 3934, Sahl bin Sa'd reports that the Companions did not count from the day the Prophet ﷺ was sent as a messenger, or from the day of his death, but from his arrival in Madinah.

The narration states the choice rather than the reasons. Historians usually explain it by what the Hijra changed: in Madinah the Muslims became a community with their own leadership. Britannica's Hijrah entry notes that after arriving, the Prophet ﷺ negotiated the Constitution of Medina with the local clans, establishing the community as a sociopolitical entity for the first time.

Why year 1 begins in Muharram

The migration took place in Rabi al-Awwal, yet year 1 begins two months earlier, on 1 Muharram. According to al-Tabari as summarised in Encyclopaedia Iranica, Umar preferred 1 Muharram, keeping the familiar Arabian year that already began with that month. Britannica's Hijrah entry states that Umar started the first year with the first day of Muharram, which corresponds to 16 July 622 in the Julian calendar.

Some tables give 15 July 622 instead, as the Iranica entry does. The difference comes from two conventions for the epoch, explained in the post on what AH means. The fuller story of the month, and the reports about who proposed it, is in the sister post on why the Hijri year starts in Muharram.

When did it happen: 16, 17 or 18 AH?

The sources agree on who and why, but not on the exact year. Encyclopaedia Iranica says early Islamic historians and later scholars are "virtually unanimous" that Umar introduced the era, and gives the year as 16, 17 or 18 AH (637–639 CE). Britannica's Hijrah entry puts it in 639 CE. The AramcoWorld calendar guide says that in 638 CE Umar recognised the need for a calendar to govern the affairs of the Muslims.

The figure most often repeated is 17 AH. Here is how those Hijri years line up with the Julian calendar used in Europe at the time:

These dates are calculated with the arithmetical (tabular) Islamic calendar using the 16 July 622 civil epoch, then converted to the Julian calendar. The months of that era were set by sighting the crescent, so the actual start of each year may differ by a day or two. The Umm al-Qura tables used elsewhere on this site do not reach back this far.

The table explains why the CE year wobbles between sources. Most of 17 AH falls in 638, but its last days fall in January 639, and 18 AH runs almost exactly through 639. A decision reported in "17 or 18 AH" can therefore be written as 638 or 639 CE without any real contradiction.

What changed after the decision

Once adopted, the Hijri era became the standard way of numbering Islamic years. Every Hijri year number since then counts from that 622 starting point, so 1448 AH, which began on 16 June 2026 on the calculated Umm al-Qura calendar, is the 1,448th year of Umar's count.

Two things did not change. The months kept their old names and their lunar length of 29 or 30 days, and the calendar stayed purely lunar, about 11 days shorter than a solar year. That is why Islamic dates move through the seasons. The post on the Hijra of 622 covers the journey that the count commemorates.

Common questions

Did the Prophet ﷺ start the Hijri calendar?

The lunar months and their sacred months predate Umar, but the numbered era did not exist as an official system in the Prophet's ﷺ lifetime. Britannica attributes the introduction of the Hijra era to Umar. Encyclopaedia Iranica notes some earlier documents that count from the Hijra, so informal use may have begun sooner.

Why is it called the Hijri calendar?

Hijri means "of the Hijra," the migration. The name records the event chosen as year 1, which is also where the abbreviation AH, anno Hegirae, comes from.

Explore the calendar Umar's decision created

The count that began with that consultation is still the one used for Ramadan, the two Eids and every other Islamic date.

• Convert any modern date between the Hijri and Gregorian calendars with the Hijri date converter.
• See the twelve months and their meanings on the Islamic months page.
• Check when the next year begins on the Islamic New Year page.

Calculated dates are a planning guide. Where months begin by local moon sighting, the announced date may differ by a day.