Recently, an innocent attempt to correct an error, in a birth date cited in a Wikipedia article, led me to a lesson in the Solar Hijri calendar, used in Iran. It was another wonderful reminder about how interesting and subtle are the calendars and clocks across cultures. Cultures can can approach the task of keeping track of days and years so differently, despite all of us living on the same planet, orbiting the same star and watching the same moon.

 This month I had the pleasure of singing in a “Persian Night” concert as part of the chorus. The main attraction was Alireza Ghorbani, a hugely famous singer of Iranian traditional and popular music. As I sometimes do, I took the opportunity to repay the artist with data. I polished Ghorbani’s page on MusicBrainz and the Alireza Ghorbani article on English-language Wikipedia. The Wikipedia article gave Ghorbani’s birth year as 1973, but his own official bio in English,, said 1972. Which was correct? And how did the Wikipedia editor who wrote 1973 get it wrong?

The clue was in Ghorbani’s official bio in Farsi, I don’t read Farsi, but my (solidly North American) computer had no trouble displaying the Farsi text, and a helpful web service gave me a quite usable translation into English. Oh brave new world, that has such tech in it! The Farsi bio gave the birth date as ‫۱۵ بهمن ۱۳۵٠‬ or “15 Bahman 1350″.

Thus it was that I got to learn about the Solar Hijri calendar. This is the calendar used in Iran. It draws from millenia-old Persian traditions. Like the Gregorian calendar I grew up with, it is aligned with the solar year. This contrasts with the lunar-aligned Hijri calendar used in many other Islamic nations. Unlike the Gregorian calendar, the Solar Hijri calendar aligns the start of the new year with the Northward (or March) equinox, the point of intersection between the ecliptic and the celestial equator. This is a really sensible idea. It is much less arbitrary than starting a new year on the arbitrary mid-winter date which January 1st is. It means many annual celestial events occur predictably on the same date each year. The year has 12 months: the first six have 31 days, the next five have 30 days, and the twelfth month has 29 days most years, but 30 days in leap years. This is more regular than the lopsided Gregorian calendar, with its seven months of 31 days, and its shriveled 28-day month.

Which years are leap years? In the Solar Hijri calendar, this is based on observation of the actual motion of the Earth and the sun. Observers in Tehran determine when the equinox actually happens. If that moment is before noon, then that day is the first of the new year. If the moment is after noon, then the next day is the first of the new year. In practice, usually every fourth year is a leap year, but occasionally it is the fifth year which is the leap year. This is in contrast to the Gregorian calendar, which has an algorithm for determining which years are leap years, and the calendar can be projected centuries into the future.

My first reaction, on learning that the length of the Solar Hijri year was determined from observation instead of from algorithm, was that it must be terribly inconvenient not to be able to predict what the date will be at some point in the future. Will the date 1000 days from now be the 7th or the 8th of Shahrivar? Growing up with the Gregorian calendar, one takes that predictability for granted. I speculate, though, that the Solar Hijri dates aren’t so mysterious in practice. There is a fairly accurate algorithm for predicting which Solar Hijri years will be leap years, based on a 33-year cycle. I imagine that calendar-makers have found ways to predict the leap years well enough to be confident in printing day-planners and releasing date-calculation software in calendars and spreadsheets. Contrariwise, the Gregorian calendar can’t be relied on for more than a century or two. Human political decisions have caused discontinuities in Gregorian-based calendar in the past, and probably will again.

But back to Mr Ghorbani. A handy Gregorian-Solar Hijri date converter at converted 15 Bahman 1350 to 4. February 1972 . Note that the month of Bahman falls in the late part of the Solar Hijri year, when it overlaps a different Gregorian year than the earlier months. I speculate that this is what led the earlier Wikipedia editor astray, causing them to enter 1973 instead of 1972. So, I was able to make a minor improvement to a Wikipedia article and a MusicBrainz entry, and repay Mr Ghorbani in a tiny measure for his beautiful singing by improving his metadata. But the real treat was a chance to learn about this wonderful calendar, and then share it.

P.S. there was a bonus — a chance to use two Unicode characters for real for the first time: U+202B RIGHT-TO-LEFT EMBEDDING, and U+202C POP DIRECTIONAL FORMATTING, which let the Farsi text above display properly in this left-to-right context, according to the Unicode Bi-di Algorithm.