Who Started the Islamic Calendar? The History Behind 1 AH

The Islamic calendar feels timeless, but it has a clear beginning. Its months are older than Islam; its year count is a decision taken in a specific meeting, by a specific caliph, for practical reasons. Knowing that history makes the calendar easier to understand — and explains some of its quirks, such as why the year begins in Muharram even though the Hijrah happened in Rabi’ al-Awwal.

Before Islam: a lunisolar calendar

Arabia before Islam used twelve lunar months with the same names we use today: Muharram, Safar, the two Rabi’s, the two Jumadas, Rajab, Sha’ban, Ramadan, Shawwal, Dhul-Qa’dah and Dhul-Hijjah. Several names describe seasons — Rabi’ (spring), Jumada (freezing), Ramadan (scorching heat) — which suggests that when they were coined, the months stayed roughly in their seasons.

To keep them there, the Arabs practised nasi’ (النَّسِيء): from time to time an extra month was added, or a sacred month was postponed, so that the pilgrimage season and trade fairs fell in convenient weather. The decision was in the hands of a particular clan, and it was also used to shift the sacred months — in which fighting was forbidden — to suit tribal politics.

There was also no universal year count. Arabs dated events by memorable happenings: “the Year of the Elephant” (the year Abraha’s army marched on Makkah, traditionally the year of the Prophet’s birth), “the year of such-and-such battle”. That works within one generation; it does not work for administration across an empire.

The Quran fixes the year at twelve lunar months

During the Prophet’s farewell pilgrimage in 10 AH (632 CE), the calendar was settled in its Islamic form. The Quran declares: “Indeed, the number of months with Allah is twelve… of them four are sacred” (9:36), and condemns the postponement: “The nasi’ is only an increase in disbelief” (9:37).

In his sermon at that pilgrimage the Prophet ﷺ said that time had “returned to its original form, as on the day Allah created the heavens and the earth”, and named the four sacred months (Sahih al-Bukhari, 3197). From then on, the year was purely lunar: twelve months, each beginning with the new crescent, and no extra month.

What the Prophet’s era did not have was a numbered year. Letters and treaties from that period are dated by month and event, not by “year 5” or “year 9”. The numbering came later.

Umar’s decision: a year count for an empire

By the time of Umar ibn al-Khattab (caliph 13–23 AH, 634–644 CE), the Muslim state stretched from Egypt to Iraq. Governors, judges and treasury officials exchanged documents constantly, and undated or ambiguously dated letters caused real problems.

Historical reports — preserved by al-Tabari and others — describe the trigger in slightly different ways. In one account, Abu Musa al-Ash’ari, governor in Basra, wrote to Umar complaining that letters arrived dated only “Sha’ban” and he could not tell which Sha’ban was meant. In another, a document came to Umar with a due date and he asked which year it referred to. The details differ; the problem is the same.

Umar gathered the companions to decide on an epoch — a “year one”. The reported proposals included:

• the birth of the Prophet ﷺ;
• the start of revelation (the first verses);
• the death of the Prophet ﷺ;
• the Hijrah to Madinah.

The view attributed to Ali ibn Abi Talib, and adopted by Umar, was the Hijrah. The birth year was uncertain; the death was a sorrowful event; the start of revelation was not a public moment. The Hijrah, by contrast, was known to everyone and marked the birth of the Muslim community as an independent society. Most historians place the decision in 17 AH (638 CE), with some reports giving 16 AH.

Why the year begins in Muharram

The Hijrah itself took place in Rabi’ al-Awwal — the Prophet ﷺ reached Quba near Madinah early in that month. Logically, the calendar could have started there. Instead, the companions kept Muharram as the first month.

The reported reasoning is that Muharram was already the customary start of the Arab year: people returned from the pilgrimage in Dhul-Hijjah, and Muharram marked a new beginning. Some reports also connect it to the Prophet’s resolve to emigrate having been formed after the pledge of Aqabah, which took place during the Hajj season. Uthman ibn Affan is among those reported to have suggested Muharram.

The result is a small but important quirk: 1 Muharram 1 AH is about two months before the Hijrah itself. It corresponds to 16 July 622 in the Julian calendar used at the time.

Spreading and standardising the calendar

Once adopted, the Hijri year count spread quickly through official use: coins, papyri and inscriptions from the early Umayyad period carry Hijri dates. Egyptian papyri are among the earliest surviving documents dated with the new era.

Because months began with the sighted crescent, different regions could be a day apart. For administration and astronomy, Muslim scholars developed arithmetical (tabular) calendars: months alternating between 30 and 29 days, with leap days in 11 years of each 30-year cycle. Astronomers such as al-Khwarizmi and al-Biruni worked on these systems and on predicting crescent visibility, and the tabular calendar is still used today by historians and in software.

The calendar in the modern era

In the 19th and 20th centuries many Muslim-majority states adopted the Gregorian calendar for civil purposes while keeping the Hijri calendar for religion. The Ottoman Empire used several calendars side by side; Turkey adopted the Gregorian calendar officially in 1926.

Saudi Arabia kept the Hijri calendar as its official civil calendar, publishing the Umm al-Qura calendar based on astronomical criteria for Makkah. In 2016 the government moved salaries and the fiscal year to the Gregorian calendar, but Hijri dates remain common on documents and in daily life.

Elsewhere, countries determine religious months in their own ways — local sighting in Pakistan, Bangladesh and Morocco; calculation-based calendars in Turkey and, for many communities, North America. The calendar is the same; the method of starting each month differs.

How the companions dated things before the year count

Even without numbered years, the early Muslim community kept careful track of time. Early historians record that each year after the Hijrah was given a name taken from its main event. The first year was “the year of permission” (to migrate), the second “the year of the command to fight”, and so on until the tenth, “the year of the farewell”. Lists of these year-names appear in later chronicles, though not every list agrees on every name.

This system worked for people who had lived through the events. It did not work for a child born in Kufa or a tax collector in Egypt who had never heard what “the year of the earthquake” referred to. The move to numbered years was therefore less a religious innovation than an administrative necessity — and the companions treated it that way, discussing it openly and reaching a consensus.

Timeline at a glance

Date	Event
Before 610 CE	Lunisolar Arab calendar with intercalation (nasi’)
16 July 622 (Julian)	1 Muharram 1 AH — the later-designated first day
September 622	The Hijrah: arrival in Madinah (Rabi’ al-Awwal 1 AH)
10 AH / 632 CE	Farewell pilgrimage; intercalation ended (Quran 9:36–37)
c. 17 AH / 638 CE	Umar ibn al-Khattab establishes the Hijri year count
9th–11th centuries CE	Tabular calendars and visibility studies by Muslim astronomers
20th century	Gregorian calendar adopted for civil use in many Muslim countries
2016	Saudi Arabia moves salaries and fiscal year to the Gregorian calendar

Why the history still matters

Three practical points follow from this history:

1. The months are fixed in number and order by the Quran, but their start dates depend on the moon — which is why calendars can differ by a day.
2. The year count is administrative, chosen by consensus of the companions. It is a shared convention, which is why the whole Muslim world uses the same year number.
3. Calculated calendars are not new. Muslims have used arithmetic and astronomy to plan ahead for more than a thousand years; today’s debate is about which method to use for worship, not whether calculation is legitimate at all.

To see how any historical Gregorian date maps onto the Hijri calendar, try the converter — and switch to the tabular method, which is the convention historians use for dates before the modern era.