I use two websites to convert dates from/to the Islamic and the Common Era calendars. You need to know about four different calendars to understand this properly.
- Hijrī calendar: the standard term for the Islamic calendar, which began counting when Muhammad went to (hajara) Medina, in 622 CE. It is often denoted by A.H., h., or هــ.
- The Hijrī calendar actually knows two flavors. The standard one divides the year into 12 months based on moon cycles. This means the total number of days is only 354 (or 355, it differs) days, in other words, 10 full days less than what we are used to. Sometimes the moon (qamr) flavor of the Hijrī calendar is indicated by Qamrī, Q, قـ, or ق.
- In Persian culture, a calendar has been adopted that starts with the Hijra, but has a length of year based on cycles around the sun. It is therefore longer than the Qamrī-Hijrī calendar. It can be indicated by Shamsī (sun), Sh, ش, or شـ.
Note: Many books from Iran that are of religious nature will have their date in Qamrī-Hijrī, not Shamsī-Hijrī.
- Mīlādī calendar: the standard term for the Christian calendar, so called because it begins counting in the year of the birth of Jesus (mīlād = birth). It is customary nowadays to not refer to this calendar as Christian, but as Common. Instead of A.D. (= Anno Domini = In the year of the Lord) one could abbreviate C.E. (= Common Era). In Arabic texts, mīlādī is indicated by م or مـ.
- Believe it or not, also the Mīlādī calendar comes in two flavors. The difference is also in calculating the total number of days. The Julian calendar was in force until 4 October 1582. At that time, the discrepancy between the calendar and the cycle around the sun became too great to hold on to it. It needed to be readjusted and so it was.
- The readjusted calendar is called Gregorian. 4 October 1582 (Julian) was followed by 15 October 1582 (Gregorian). Our Common Era follows the Gregorian calendar until this day.
I use two websites for converting dates.
- For ordinary dates, I use http://www.iranchamber.com/calendar/converter/iranian_calendar_converter.php This gives Shamsī, Qamrī, and Gregorian dates. You can adjust any text box or drop-down box, hit the corresponding ‘calculate’, and the other boxes will update. Dates before 15 October 1582 are not corrected to Julian calendar. For example, for 28 Rabi al-Thani 600, it gives 11 January 1204. However, Julian calendar gives 4 January 1204.
- For more precise conversions I use http://www.staff.science.uu.nl/~gent0113/islam/islam_tabcal.htm (click on ‘Islamic calendar converter’). As you can see, it is very precise, bordering at the too precise. Also, it is more tedious to use because you can only operate it by using the buttons at the bottom. Changing the text boxes or drop-down boxes does not do anything. This converter automatically changes to Julian.
What is a correct date? It has become more and more acceptable to use Gregorian calendar, even for dates before 15 October 1582. In reference to our own times, this is of course reasonable. However, those who are working with primary sources both from Europe and the Islamic world should be aware that when an Islamic source has 28 Rabi al-Thani 600, the corresponding date in European sources is 4 January 1204. Whatever you do, it will be a great service to your readers to indicate which calendar you are following.