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Hijri Dates and Why Two Books Give Different Years for One Event

An AH year is about 11 days shorter than a CE year, so it straddles two of them. Here are the four reasons careful sources still convert it differently.

By Published 5 min read

Reign Creative LLC makes World History Timeline Sim, which this article links to, so we have a commercial interest in it. How we write and check articles.

Key takeaways

  • Twelve lunar months run about 11 days shorter than a Gregorian year, so a single AH year usually overlaps two CE years and a year-only Hijri date converts to a range rather than a point.
  • The epoch itself has two accepted values: astronomers generally use Thursday 15 July 622 in the Julian calendar, while chronological tables generally use Friday 16 July 622.
  • Historians working on Islamic chronology commonly use a tabular calendar with a 30-year cycle containing 11 leap years, which is regular by design and can differ by a day from an observed month start.
  • Month beginnings depend in principle on sighting the new crescent with the unaided eye, and there is open disagreement about whether to rely on observation or on calculation.
  • Two reputable books can therefore give different Common Era years for one event without either being wrong, and the difference is a convention, not an error.

A Hijri year runs about eleven days shorter than a Gregorian one, so almost every AH year begins in one Common Era year and ends in the next. That single fact means a Hijri date given only to the year cannot be converted to a single CE year at all — the honest answer is a span of two. Add three further points where reputable authorities legitimately differ, and you have a complete explanation for why two well-edited books can print different Common Era years for the same event.

What the calendar is

The Islamic calendar is purely lunar, with twelve months and no intercalated extra month to keep it aligned with the solar year. The US Naval Observatory describes months as beginning when the thin crescent Moon is actually sighted in the western sky after sunset, a day or so after the new moon, and notes that for calendar purposes the sighting must be made with the unaided eye.

Years are counted from the Hijra — the migration of the Prophet Muhammad and his followers from Mecca to Medina — and are marked AH. The standard reference account of calendars credits the caliph ʿUmar I with establishing the era in AH 17, and notes that the form of the calendar as a lunar calendar without intercalation was laid down in the Qurʾan and in the Prophet's sermon at the Farewell Pilgrimage. The Islamic day begins at sunset of the previous day.

Why one AH year maps onto two CE years

Twelve lunar months are, on average, eleven days shorter than the Gregorian civil year. So each Islamic year begins about eleven days earlier in the civil year than the one before, and the whole cycle regresses through the seasons over roughly 33 years.

The practical consequence for historical writing is immediate. If a chronicle says only "in AH 350", the corresponding Common Era period straddles a boundary, and a writer who picks one of the two years has made a silent choice. Some make it consistently, some make it silently, and different books make it differently.

The four places a conversion can legitimately differ

1. The epoch. There are two accepted starting points, one day apart. The reference account of calendars records that 1 Muharram AH 1 is generally taken by astronomers as Thursday, 15 July 622 in the Julian calendar — the astronomical Hijra epoch — while chronological tables generally use Friday, 16 July 622, the civil epoch. The US Naval Observatory gives the civil value.

2. Observed versus tabular months. The religious calendar depends on sighting. Chronologists working on historical material generally use a tabular calendar instead: a regular scheme with a thirty-year cycle in which eleven years are leap years, odd-numbered months of thirty days and even-numbered months of twenty-nine, with an extra day added to the final month in a leap year. The tabular scheme is reproducible and can be a day away from what was actually observed at the time.

3. Julian or Gregorian on the other side. For dates before the Gregorian reform, "the CE equivalent" depends on which Western calendar is meant. That difference grows over time and is separate from anything to do with the Hijri calendar. How calendars and dating systems work covers the reform and why a naive interval across it is wrong.

4. How much of the date survives. A full day-month-year Hijri date converts to a single day. A year alone converts to a two-year span. Sources rarely mark which situation they were in.

The crescent problem is not a historical curiosity

Deciding when a lunar month begins is an active technical question, not a solved one.

The reference account of calendars puts it bluntly: because of weather, a new month may be declared thirty days after the start of the previous one, various predictive procedures for first visibility have always had an equivocal status, and there is disagreement among countries, religious leaders and scientists about whether to rely on observations, which are subject to error, or on calculations, which may rest on poor models.

The astronomy behind this continues to be refined. A study in Experimental Astronomy established a visibility criterion from 737 observations — almost half of them collected by the Islamic Crescent Observation Project — based on the topocentric arc of vision and the topocentric crescent width, covering both naked-eye and optically aided sightings, and reporting a Danjon limit of 6.4 degrees. That is a live measurement programme producing new criteria, which is exactly why historical month starts cannot simply be back-calculated with confidence.

Where the dates come from in the first place

Many Islamic-world dates reach us through manuscripts that record their own completion, usually in the colophon at the end — the note in which a copyist states what was copied, by whom, and when. The colophon is a genuine object of scholarly study in its own right, examined in the Journal of Islamic Manuscripts among other venues, and digitised collections such as the Cambridge Digital Library's Islamic Manuscripts holdings let a reader look at the primary object rather than only at a catalogue entry.

This matters for dating because a colophon date is a date for the copy, not for the text. Treating one as the other is a common error, and it is the same category of mistake as reading a regnal date without knowing the accession convention. How historians date events covers the general version of that problem.

A rough conversion, honestly labelled

Because the lunar year slips a full year against the solar year roughly every 33 years, a serviceable mental approximation is: take the AH year, subtract the AH year divided by 33, and add 622. The result is the Common Era year in which that Hijri year began, give or take one.

Use it for orientation. Do not use it for citation. For anything you intend to publish, convert from the full Hijri date with a tool that states its epoch and its calendar, and say in your text which convention you used.

What this means for a timeline

Any chronology tool showing a Common Era year for an early Islamic event is displaying a conversion somebody else performed, with the four choices above already made and invisible.

World History Timeline Sim is our own app, built by Reign Creative, and it is genuinely useful for seeing how the early caliphates, the Fatimids, the Seljuks, the Mamluks, the Ottomans and the Safavids sit relative to one another and to everything else happening at the same time. That relative shape is what a timeline is good at. For a specific date you intend to rely on, go to the manuscript catalogue or the scholarly edition — the app is an exploration tool, not a substitute for primary sources or peer-reviewed work, and a converted date is precisely the kind of claim worth checking.

The wider period this sits inside is covered in what late antiquity means, and orientation on using a chronology well in how to actually learn something from a timeline app. More is collected in the world history timelines hub, and our other learning tools under education and brain games.

Good to know

Frequently asked questions

What does AH mean in a date?
It marks a year counted from the Hijra, the migration of the Prophet Muhammad and his followers from Mecca to Medina, which is the epoch of the Islamic calendar. The US Naval Observatory gives 1 Muharram AH 1 as corresponding to Friday 16 July 622 CE in the Julian calendar. A year labelled AH is therefore not comparable with a CE year of the same number by simple subtraction.
Why does an Islamic date move through the seasons?
Because the calendar is purely lunar with no intercalated month. Twelve lunar months average about 11 days fewer than a solar year, so each Islamic year starts earlier in the civil year than the last. The US Naval Observatory notes that the cycle of twelve months regresses through the seasons over a period of about 33 years.
Is there a formula for converting AH to CE?
There is a rough one, and it should be treated as rough. Because the lunar year loses roughly one year against the solar year every 33 years, the Common Era year in which an AH year begins is approximately the AH year, minus the AH year divided by 33, plus 622. That gets you the right year or one adjacent to it, which is fine for orientation and not good enough for citation.
Which conversion should I use when writing?
Give the Hijri date as your source gives it, then the Common Era equivalent, and say which calendar the equivalent is in — Julian or Gregorian. If your source supplies only an AH year with no month, write the Common Era result as a span of two years rather than choosing one. Stating the convention costs a clause and removes the ambiguity entirely.

Sources

  1. The Islamic Calendar (opens in a new tab)Astronomical Applications Department, U.S. Naval Observatory — accessed
  2. Calendars and their History (opens in a new tab)NASA Goddard Space Flight Center (reprinted from the Explanatory Supplement to the Astronomical Almanac) — accessed
  3. New Criterion for Lunar Crescent Visibility (opens in a new tab)Experimental Astronomy (Springer Nature) — accessed
  4. The Colophon in Arabic Manuscripts: A Phenomenon without a Name (opens in a new tab)Journal of Islamic Manuscripts (Brill) — accessed
  5. Islamic Manuscripts (opens in a new tab)Cambridge Digital Library, University of Cambridge — accessed

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