Why Is 622 CE the First Year of the Islamic Calendar?

The short answer

The Islamic calendar counts years from the Hijra — the migration of the Prophet Muhammad and the early Muslim community from Mecca to Medina. The lunar year in which that migration took place was designated year 1, and that year began in 622 CE on the Gregorian scale. The U.S. Naval Observatory places the epoch of the Islamic calendar in 622 CE and notes that the lunar year runs about 11 days shorter than the Gregorian one (USNO).

In one line: 622 is not a birth year or a death year — it is the Gregorian year in which Hijri year 1 opened.

What is being dated

Eras are usually anchored to an event that a whole community can date without dispute. The Hijra qualifies: it marks the relocation of a community and the start of its organized life in Medina. The calendar does not begin at the moment of the journey itself but at the start of the lunar year containing it — 1 Muharram, year 1, which falls in 622 CE.

• The anchor is communal, not biographical. That is why the era is named for a migration rather than for a person.
• The anchor is retroactive. The era was adopted later and applied backward, the same way the Common Era was.
• The anchor is lunar. Year 1 is a lunar year of 354 or 355 days, not a solar one.

Summary: year 1 AH is a lunar year that opened in 622 CE.

Why 622 + a number does not give you the Hijri year

This is the single most common arithmetic mistake. Because the Hijri year is shorter, the two calendars slide against each other:

• A Hijri year of 354–355 days against a Gregorian year of 365–366 days loses roughly 11 days annually (USNO).
• Over about 33 Hijri years, that accumulates to a full extra year relative to the Gregorian count.
• So the Gregorian year is not 621 or 622 plus the Hijri year. The gap narrows steadily as the centuries pass.

The reverse direction has the same trap: a single Hijri year overlaps two Gregorian years, and a single Gregorian year overlaps two Hijri ones. For the correct method and why tools disagree at the edges of a month, see how to convert a Hijri date to a Gregorian date.

Summary: the offset between the two calendars is a moving number, not a constant.

Why you see slightly different dates for 1 Muharram 1 AH

Two honest reckonings exist side by side:

• The calculated (tabular) calendar assigns month lengths by arithmetic rule, giving a definite epoch date in July 622 that software can compute backward from. The USNO's Introduction to Calendars explains why purely lunar calendars behave differently from solar and luni-solar ones.
• The sighting-based calendar starts each month from an observed crescent, so a historical reconstruction can land a day either side of the tabular value.

For Saudi administrative dating today, the reference point is the official Umm al-Qura calendar rather than a generic computed table — the distinction covered in civil Hijri vs Umm al-Qura dates.

Summary: 622 is firm; the exact day of 1 Muharram 1 AH depends on which reckoning you use.

How the era is written

Hijri years carry AH (anno Hegirae, "in the year of the Hijra"), and Gregorian years in Muslim-majority contexts are often written CE rather than AD. Both conventions, and how dual dates are formatted on documents, are covered in what AH means and do Muslims use BC and AD.

Summary: 622 CE is the Gregorian stamp on a lunar era that starts with a migration — and the two calendars have been drifting apart by about 11 days a year ever since.