The Persian calendarگاه‌شماری

Most calendars begin their year on a date fixed by custom. The Persian calendar begins it with an event in the sky. The new year, Nowruz (literally 'new day'), falls on the day of the March equinox, the moment the sun crosses the celestial equator heading north and the northern hemisphere tips into spring.

The calendar in official use in Iran today is called the Solar Hijri calendar, or Hejri Shamsi ('solar Hijri'). Its first day, 1 Farvardin, is set by a simple rule. Astronomers calculate the instant of the equinox and compare it with noon on the meridian of Iran Standard Time, 52.5 degrees east, which runs close to Tehran. If the equinox comes before that noon, the day is 1 Farvardin. If it comes after, the new year starts the following day.

Take the year we are in. In 2026 the equinox fell on the evening of 20 March, Tehran time, after noon had passed. So 1 Farvardin 1405 was Saturday 21 March 2026. In most years Nowruz lands on 20 or 21 March in the Gregorian calendar.

Families mark the precise minute of the equinox, the tahvil-e sal ('turning of the year'), and the time is published in advance down to the second. People gather round the table, watch the clock, and greet the year when the sun says so, not when midnight does.

The year has twelve months in two neat blocks. The first six, from Farvardin to Shahrivar, have 31 days each. The next five, from Mehr to Bahman, have 30. The last month, Esfand, has 29 days in a common year and 30 in a leap year. That gives 365 or 366 days in all.

The shape is not arbitrary. The sun appears to move a little more slowly across the sky in the northern spring and summer, because the Earth is near the far point of its orbit then. The interval from the March equinox to the September equinox is therefore several days longer than the return journey. Giving the first half of the year the long months lets the calendar follow the seasons closely. In practice, the first day of Mehr falls around the September equinox, and the first day of Dey around the winter solstice.

Leap years work differently from the Gregorian kind. There is no rule of 'every fourth year, except'. A year is a leap year when the next Nowruz falls 366 days after this one, which is decided by where the equinox actually lands. In that case Esfand gains its thirtieth day. Leap years usually come four years apart, and every so often the gap stretches to five.

For people who need to compute dates without an astronomical table, there are arithmetic schemes that mimic this pattern. The best-known uses a cycle of 33 years containing 8 leap years. These approximations agree with the astronomical rule for long stretches, but they are models of the calendar rather than the calendar itself.

The idea of starting the year at the equinox is old, but its modern form goes back to a reform of the eleventh century. Under the Seljuk sultan Malik-Shah, a group of astronomers was asked to fix a calendar that had drifted away from the seasons. Among them was Omar Khayyam, now better known abroad as a poet. Their result, named the Jalali calendar after one of the sultan's titles, came into use from 1079 CE and tied the new year to the spring equinox.

Why does that matter? The year the seasons follow is the tropical year, the time the sun takes to return to the same equinox. It is not a whole number of days, and it is a little shorter than 365 and a quarter. Any calendar built on a fixed counting rule has to approximate that awkward fraction.

The Gregorian calendar does it with 97 leap days every 400 years, an average year of 365.2425 days. That is close, but slightly longer than the year the equinoxes keep, so the error builds up slowly and the equinox creeps earlier over the centuries until a correction is needed. A 33-year cycle with 8 leap years averages about 365.2424 days, which is nearer still.

The Solar Hijri calendar goes one step further. Because it takes its new year from the observed equinox each time, it has no average to drift away from. The error cannot pile up, because the sky resets it every spring. The trade-off is that you need an astronomer, or an almanac, to know in advance when a leap year falls.

The years are numbered from the Hijra, the migration of the Prophet Muhammad from Mecca to Medina in 622 CE. That is the same starting point as the Islamic lunar calendar, which Muslims use for religious dates. The difference is in what is being counted. The lunar Hijri calendar counts years of twelve moons, about eleven days shorter than a solar year. The Solar Hijri calendar counts solar years. Over the centuries the lunar count has pulled ahead, which is why the lunar year now in progress is numbered in the 1440s while the solar one is 1405.

Year 1 of the Solar Hijri calendar began at the spring equinox of 622. To turn a Persian year into a Gregorian one, add 621 for dates from Nowruz to the end of December, and 622 for dates from January to the end of Esfand.

The calendar in its present form was made law by Iran's parliament in 1925, in a law dated 11 Farvardin 1304. That law gave the months the Persian names used today and fixed their lengths at 31, 30 and 29 or 30 days. Apart from a short spell in the late 1970s, when an imperial era counted from Cyrus the Great replaced the Hijri numbering, it has been Iran's civil calendar ever since.

Afghanistan also uses the Solar Hijri calendar, with the same year numbers, but names its months differently. In Dari, the Afghan form of Persian, the months take the Arabic names of the signs of the zodiac: Hamal (Aries) for the first month, and so on round to Hut (Pisces).

The Persian month names are far older than the calendar that now carries them. They come from the calendar of Zoroastrian Iran, and most of them are worn-down forms of names in Avestan, the ancient language of the Zoroastrian scriptures. Several honour the Amesha Spentas ('bounteous immortals'), the six divine qualities that stand around Ahura Mazda, the wise lord of Zoroastrian belief.

So Ordibehesht recalls Asha Vahishta, 'best truth', the right order of the world. Khordad is Haurvatat, wholeness, and Mordad is Ameretat, immortality. Shahrivar is Khshathra Vairya, 'desirable dominion'; Bahman is Vohu Manah, 'good mind'; Esfand is Spenta Armaiti, holy devotion, long linked with the earth. The others belong to the wider Zoroastrian world: the fravashis, or guardian spirits, in Farvardin; Tishtrya, the rain-bringing star, in Tir; Mithra, the guardian of covenants, in Mehr; the waters in Aban; fire in Azar; and the Creator in Dey.

In the older calendar each of the thirty days of the month also had a name, and many were the same names as the months. When a day's name and its month's name coincided, it was a feast. Tir day in the month of Tir gave Tirgan; Mehr day in the month of Mehr gave Mehregan. The modern calendar has rearranged the days, but the dates of these festivals still carry the memory of that double naming.

That is the quiet pleasure of the Persian calendar. A date in an official letter from Tehran is, underneath, a small piece of very old poetry.