Hijri and Gregorian dates in this atlas
Every date in this atlas is Common Era - Gregorian years, counted from the same zero point a Western reference uses. None of the 153 figure pages prints a Hijri year. If you need one, you have to convert, and the conversion is arithmetic you can do in your head to within a year: subtract 622 from the CE year, then add about 3 per cent.
Nubala · Published · 7 min read
The reason it is only ever about a year is structural rather than sloppy. The Hijri calendar is lunar: twelve lunar months, 354 or 355 days, roughly eleven days shorter than a solar year. So a Hijri year never lines up with a Gregorian one. Any single Hijri year begins inside one Gregorian year and ends inside the next, and any single Gregorian year contains the end of one Hijri year and the start of another. A year-to-year conversion therefore has two right answers, and a date-to-date conversion needs tables rather than arithmetic.
The conversion, both directions
The Hijri era begins with the migration from Mecca to Medina, conventionally placed in 622 CE; 1 AH is the year that began with it. Because the lunar year is about 11 days short, 33 Hijri years pass in roughly 32 Gregorian years, and that ratio is the whole arithmetic.
From CE to AH: subtract 622, then multiply by 33/32 - that is, add about one year in every thirty-two. From AH to CE: subtract one year in every thirty-three, then add 622. Expect to be within a year either way, and expect the error to be a whole year rather than a few days, because the two calendars never share a new year.
A worked example from the atlas. Fatima al-Fihri founded the mosque and school at Fez that became al-Qarawiyyin, and her page dates the founding to 859 CE. Convert: 859 minus 622 is 237; 237 plus 237/32, about 7, gives roughly 244. The Hijri year normally given for that founding is 245 AH. One year out, in the direction the method predicts.
A second, four centuries later. Al-Khwarizmi is dated about 780 to about 850 CE. That converts to roughly 163 to 235 AH - so his career sits in the second and early third centuries AH, which is where a catalogue or bibliography written in Hijri years will index him. Converting changes nothing about the man and everything about where he sits on someone else’s shelf.
| Figure | Atlas date (CE) | Approximate AH | Why it matters |
|---|---|---|---|
| Fatima al-Fihri | al-Qarawiyyin founded 859 | about 244-245 | Chronicles of Fez date the founding in AH |
| Al-Khwarizmi | about 780 to about 850 | about 163 to 235 | Places him in the 2nd-3rd centuries AH |
| Al-Ghazali | 1058 to 1111 | about 450 to 505 | His own works are dated in AH |
| Suleiman the Magnificent | 1494 to 1566 | about 899 to 973 | Ottoman documents are dated in AH |
Why a Hijri age is larger than a Gregorian one
Because a lunar year is shorter, a lifetime measured in Hijri years is about 3 per cent longer than the same lifetime measured in Gregorian years. Al-Ghazali lived 1058 to 1111 CE - 53 Gregorian years, and about 55 Hijri ones. Ibn Sina, about 980 to 1037, lived 57 Gregorian years and roughly 59 Hijri.
This is the most common quiet error when a premodern source gives an age at death. If a chronicle says a scholar died aged sixty, that is sixty lunar years, which is about fifty-eight solar ones - and if you work his birth year back from a CE death year using the reported age, you will be out by two.
The same ratio is why a Hijri century is not a Gregorian century. The 10th century AH runs from about 1494 to about 1591 CE: 97 Gregorian years, not 100. Suleiman the Magnificent's entire reign falls inside it.
What the tilde on 59 birth years is doing
Fifty-nine of the 153 birth lines in this atlas open with a tilde, and 30 of the death lines do; 62 figures carry at least one approximate date. The tilde is not a calendar marker. It means the year itself is a scholarly estimate rather than a record, and it is independent of the conversion question above - converting an estimate gives you an estimated Hijri year.
The atlas also carries three softer forms. A decade, as on Lubna of Cordoba, whose birth is given as ~940s CE. A qualified place, as on Tariq ibn Ziyad, whose death reads ~720 CE, Damascus (uncertain). And one case where the printed date is missing entirely: Sunan Kalijaga's death line reads Unknown, although the atlas still holds a year for him internally so that he can be placed on the timeline at all.
Read the tilde as a warning about which century, not which calendar. Where a figure's date is approximate, converting it to AH adds a second layer of approximation on top of the first, and the honest form of the answer is a decade rather than a year.
- 67 of the 153 birth lines are written as a year with CE; 27 are a bare year; 59 carry a tilde.
- No date in the atlas is BCE: the corpus starts with Tariq ibn Ziyad, about 670 CE.
- The corpus ends in 1981, with the death of Hamka in Indonesia - about 1,300 years in all.
- A Hijri year spans parts of two Gregorian years, so a year conversion has two defensible answers.
When arithmetic is not enough
The 33/32 method converts years. It cannot convert dates, and you should not use it to. Three reasons, each of which bites in premodern work.
First, the Hijri calendar as actually used was observational: a month began when the new crescent was sighted, which varied by place and weather, so two cities could be a day apart. Modern tabular Hijri calendars are an arithmetic reconstruction, not what a chronicler in Baghdad was using.
Second, Gregorian itself only starts in 1582, and only where it was adopted. For anything earlier, a CE date in a modern reference has normally been converted into the Julian calendar's reckoning or into a proleptic Gregorian one, and which of the two is in use is rarely stated. For a year, the difference does not show; for a day, it does.
Third, several regions in this atlas kept other calendars alongside the Hijri one. Persian and Mughal administration used solar reckonings for revenue; the Ottoman state used a solar fiscal calendar as well as the Hijri religious one. A date in an Ottoman or Mughal document may be in any of them.
So: use the arithmetic for a century and a rough year. For a specific day, use a Hijri-Gregorian conversion table, and say in your citation which calendar the original source used.
Reading an era range in two calendars
Each of the 12 era hubs prints a Gregorian range. Converting them is a quick way to see the shape of the corpus the way a premodern bibliography would.
The Golden Age is given as about 750 to 1300 CE, which is roughly 133 to 700 AH. Al-Andalus runs 711 to 1492 CE, about 92 to 897 AH - so the fall of Granada, 1492 CE, lands just short of the Hijri year 900, and Andalusi chroniclers writing afterwards are working in the 10th century AH. The Ottoman Empire is 1299 to 1922 CE, roughly 699 to 1341 AH.
Those three conversions are also a usable memory aid: Hijri 100 is around 718 CE, Hijri 500 around 1106, Hijri 1000 around 1591, and Hijri 1300 around 1882. Anchor on those four and you can place any Hijri year in the atlas within a few decades without arithmetic at all.
Questions readers ask
Does Nubala show Hijri dates?
No. Every one of the 153 figure pages prints Gregorian CE years only, and the timeline orders figures by a CE year. The conversion in this guide - subtract 622, add about 3 per cent - gets you the Hijri year to within one.
Why is the conversion only accurate to a year?
Because a lunar Hijri year is about eleven days shorter than a solar one, so the two calendars never share a new year. Every Hijri year straddles two Gregorian years and vice versa, which means a year-level conversion legitimately has two answers.
How do I convert a Hijri year to CE?
Subtract one year for every thirty-three, then add 622. For 500 AH: 500 minus about 15 is 485, plus 622 gives about 1107 CE - which is inside the life of Al-Ghazali, 1058 to 1111.
Why does a chronicle give a different age at death than the atlas implies?
Usually because the chronicle counts lunar years. A lifetime in Hijri years is about 3 per cent longer than the same lifetime in Gregorian years, so a reported age of sixty is about fifty-eight solar years.
What does the tilde on a date mean?
That the year is an estimate, not a record. Fifty-nine birth lines and 30 death lines carry one, and 62 of the 153 figures have at least one approximate date. It is independent of the calendar question: an approximate CE year converts to an approximate Hijri year.
Sources
- F. A. Shamsi, "The Date of Hijrah," Islamic Studies 23, no. 3 (1984).
- G. S. P. Freeman-Grenville, The Islamic and Christian Calendars, AD 622-2222 (AH 1-1650): A Complete Guide for Converting Christian and Islamic Dates and Dates of Festivals (Garnet, 1995).
- Jonathan Bloom and Sheila Blair, The Grove Encyclopedia of Islamic Art and Architecture (Oxford University Press, 2009) - cited on the Fatima al-Fihri page for the founding of al-Qarawiyyin.
- Roshdi Rashed, Al-Khwarizmi: The Beginnings of Algebra (Saqi Books, 2009) - cited on the Al-Khwarizmi page.
- Halil Inalcik, The Ottoman Empire: The Classical Age, 1300-1600 (Weidenfeld & Nicolson, 1973) - cited across the Ottoman figure pages.
Counts in this guide are derived from the atlas data itself, so the prose cannot drift from the corpus. How to read the sources a figure page cites explains how the bibliographies are built and what each kind of work can carry.