Iranian calendar

The Iranian calendars or Persian calendars ( Gâhšomâri-ye Irâni) are a succession of calendars invented or used for over two millennia in Greater Iran. One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes.

The modern Iranian calendar (Solar Hejri) is now the official calendar in Iran and Afghanistan. It begins on the vernal equinox as determined by astronomical calculations for the Iran Standard Time meridian (52.5°E or GMT+3.5h). This determination of starting moment is more accurate than the Gregorian calendar because it is synchronized with the vernal equinox year, but requires consulting an astronomical almanac. Its years are designated AP, short for Anno Persico. The Iranian year usually begins within a day of 21 March of the Gregorian calendar. To find the corresponding year of the Gregorian calendar, add 621 or 622 (depending on the time of the year) to a Solar Hejri year. A short table of year correspondences between the Persian and Gregorian calendars is provided below.

Ancient calendars
Although the earliest evidence of Iranian calendrical traditions is from the second millennium BC, predating the appearance of the Iranian prophet Zoroaster, the first fully preserved calendar is that of the Achaemenids. Throughout recorded history, Persians have been keen on the idea and importance of having a calendar. They were among the first cultures to use a solar calendar and have long favoured a solar over lunar and lunisolar approaches. The sun has always been a symbol in Iranian culture and is closely related to the folklore regarding Cyrus the Great himself.

Old Persian calendar
Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the Babylonian system (the Babylonian Calendar was lunar) and modified for their beliefs. Days were not named. The months had two or three divisions depending on the phase of the moon. Twelve months of 30 days were named for festivals or activities of the pastoral year. A 13th month was added every six years to keep the calendar synchronized with the seasons.

The following table lists the Old Persian months.

Zoroastrian calendar
The first calendars based on Zoroastrian cosmology appeared in the later Achaemenid period (650 to 330 BCE). They evolved over the centuries, but month names changed little until now.

The unified Achaemenid empire required a distinctive Iranian calendar, and one was devised in Egyptian tradition, with 12 months of 30 days, each dedicated to a yazata (Eyzad), and four divisions resembling the Semitic week. Four days per month were dedicated to Ahura Mazda and seven were named after the six Amesha Spentas. Thirteen days were named after Fire, Water, Sun, Moon, Tiri and Geush Urvan (the soul of all animals), Mithra, Sraosha (Soroush, yazata of prayer), Rashnu (the Judge), Fravashi, Bahram (yazata of victory), Raman (Ramesh meaning peace), and Vata, the divinity of the wind. Three were dedicated to the female divinities, Daena (yazata of religion and personified conscious), Ashi (yazata of fortune) and Arshtat (justice). The remaining four were dedicated to Asman (lord of sky or Heaven), Zam (earth), Manthra Spenta (the Bounteous Sacred Word) and Anaghra Raocha (the 'Endless Light' of paradise).

The month names and their modern versions are given in the following table.

The calendar had a significant impact on religious observance. It fixed the pantheon of major divinities, and also ensured that their names were uttered often, since at every Zoroastrian act of worship the yazatas of both day and month were invoked. It also clarified the pattern of festivities; for example, Mitrakanna or Mehregan was celebrated on Mithra day of Mithra month, and the Tiri festival (Tiragan) was celebrated on Tiri day of the Tiri month.

After the conquests by Alexander of Macedon and his death, the Persian territories fell to one of his generals, Seleucus (312 BCE), starting the Seleucid dynasty of Iran. Based on the Greek tradition, Seleucids introduced the practice of dating by era rather than by the reign of individual kings. Their era became known as that of Alexander, or later the Seleucid era. Since the new rulers were not Zoroastrians, Zoroastrian priests lost their function at the royal courts, and so resented the Seleucids. Although they began dating by eras, they established their own era of Zoroaster.

That was the first serious attempt to determine the dates associated with the prophet Zoroaster's life. Priests had no Zoroastrian historical sources, and so turned to Babylonian archives famous in the ancient world. From these they learned that a great event in Persian history took place 228 years before the era of Alexander. In fact, this was the conquest of Babylon by Cyrus the Great in 539 BCE. But the priests misinterpreted this date to be the time the "true faith" was revealed to their prophet, and since Avestan literature indicates that revelation happened when Zoroaster was 30 years old, 568 BCE was taken as his year of birth. The date entered written records as the beginning of the era of Zoroaster, and indeed, the Persian Empire. This incorrect date is still mentioned in many current encyclopedias as Zoroaster’s birth date.

Modifications by Parthians, Ardashir I, Hormizd I, Yazdgerd III
The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BCE, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians. For example in Achaemenid times the modern Persian month ‘Day’ was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).

In 224 CE, Ardashir I, founder of the Sassanid dynasty, added five days at the end of the year, and named them ‘Gatha’ or ‘Gah’ days after the ancient Zoroastrian hymns of the same name. This was a modification of the 365-day calendar adopted by Julius Caesar in 46 BCE, based on the Egyptian solar calendar. Iranians had known about the Egyptian system for centuries but never used it. The new system created confusion and met resistance. Many rites were practiced over many days to make sure no holy days were missed. To this day many Zoroastrian feasts have two dates.

To simplify the situation, Ardeshir’s grandson, Hormizd I, linked the new and old holy days into continual six-day feasts. Nowruz was an exception, as the first and the sixth day of the month were celebrated separately, and the sixth became more significant as Zoroasters’ birthday. But the reform did not solve all the problems, and Yazdgerd III, the last ruler, introduced the final changes. The year 632 was chosen as the beginning of a new era, and this last imperial Persian calendar is known as the Yazdgerdi calendar.

Medieval era : Jalali calendar
Before the Yazdgerdi calendar was completed, Muslim Arabs overthrew the dynasty in the 7th century and established the Islamic calendar, a lunar calendar. It was outlined in the Qur'an, and in the last sermon of Muhammad during his farewell pilgrimage to Mecca. Umar, the second caliph of Islam, began numbering years in AH 17 (638 CE), regarding the first year as the year of Muhammad's Hijra (emigration) from Mecca to Medina, in September 622 CE. The first day of the year continued to be the first day of Muharram. Years of the Islamic calendar are designated AH from the Latin Anno Hegirae (in the year of the Hijra).

The solar Jalali calendar was adopted on 15 March 1079 by the Seljuk Sultan Jalal al-Din Malik Shah I (for whom it was named), based on the recommendations of a committee of astronomers, including Omar Khayyam, at the imperial observatory in his capital city of Isfahan. Month computations were based on solar transits through the zodiac, a system integrating ideas taken from Hindu calendars. Later, some ideas from the Chinese-Uighur calendar (1258) were also incorporated. It remained in use for eight centuries. It arose out of dissatisfaction with the seasonal drift in the Islamic calendar which is due to that calendar being lunar instead of solar. Twelve lunar cycles occur over 354 days. Sultan Jalal commissioned the task in 1073. Its work was completed well before the Sultan's death in 1092, after which the observatory would be abandoned.

The year was computed from the vernal equinox, and each month was determined by the transit of the sun into the corresponding zodiac region, a system that incorporated improvements on the ancient Indian system of the Surya Siddhanta (Surya=solar, Siddhanta=analysis, 4th century), also the basis of most Hindu calendars. Since the solar transit times can have 24-hour variations, the length of the months vary slightly in different years (each month can be between 29 and 32 days). For example, the months in two last years of the Jalali calendar had:
 * 1303 AP: 30, 31, 32, 31, 32, 30, 31, 30, 29, 30, 29, and 30 days,
 * 1302 AP: 30, 31, 32, 31, 31, 31, 31, 29, 30, 29, 30, and 30 days.

Because months were computed based on precise times of solar transit between zodiacal regions, seasonal drift never exceeded one day, and also there was no need for a leap year in the Jalali calendar. However, this calendar was very difficult to compute; it required full ephemeris computations and actual observations to determine the apparent movement of the Sun. Some claim that simplifications introduced in the intervening years may have introduced a system with eight leap days in every cycle of 33 years. (Different rules, such as the 2820-year cycle, have also been accredited to Khayyam). However, the original Jalali calendar based on observations (or predictions) of solar transit would not have needed either leap years or seasonal adjustments.

In 1079, the team also computed the length of a solar year as 365.24219858156 days. (The actual value was 365.2422464 days).

However, owing to the variations in month lengths, and also the difficulty in computing the calendar itself, the Iranian calendar was modified to simplify these aspects in 1925 (1304 AP).

In Iran
On 21 February 1911, the second Persian parliament adopted as the official calendar of Iran the Jalālī solar calendar with months bearing the names of the twelve constellations of the zodiac and the years named for the animals of the duodecennial cycle; it remained in use until 1925. The present Iranian calendar was legally adopted on 31 March 1925, under the early Pahlavi dynasty. The law said that the first day of the year should be the first day of spring in "the true solar year", "as it has been" (کماکان). It also fixed the number of days in each month, which previously varied by year with the tropical zodiac. It revived the ancient Persian names, which are still used. It specified the origin of the calendar (Hegira of Muhammad from Mecca to Medina in 622 CE). It also deprecated the 12-year cycles of the Chinese-Uighur calendar which were not officially sanctioned but were commonly used.

The first six months (Farvardin–Shahrivar) have 31 days, the next five (Mehr–Bahman) have 30 days, and the last month (Esfand) has 29 days or 30 days in leap years. The reason the first six months have 31 days and the rest 30 may have to do with the fact that the sun moves slightly more slowly along the ecliptic in the northern spring and summer than in the northern autumn and winter (the time between the vernal equinox and the autumnal equinox is about 186 days and 10 hours, the opposite duration about 178 days, 20 hours).

The Solar Hejri calendar produces a five-year leap year interval after about every seven four-year leap year intervals. It usually follows a 33-year cycle with occasional interruptions by single 29-year or 37-year subcycles. By contrast, some less accurate predictive algorithms are suggestion based on confusion between average tropical year (365.2422 days, approximated with near 128-year cycles or 2820-year great cycles) and the mean interval between spring equinoxes (365.2424 days, approximated with a near 33-year cycle).

In 1976, Shah Mohammad Reza Pahlavi changed the origin of the calendar, using the birth of Cyrus as the first day, rather than the Hejra of Muhammad. Overnight, the year changed from 1355 to 2535. The change did not last however as it was "largely ignored".

In Afghanistan
Afghanistan legally adopted the official Jalali calendar in 1922 but with different month names. The Persian language in Afghanistan uses Arabic names of the zodiac signs. The Pashto language in Afghanistan uses the Pashto names of the zodiac signs. The Persian calendar is the official calendar of the government of Afghanistan, and all national holidays and administrative issues are fixed according to the Persian calendar.

Details of the modern calendar
The Solar Hejri calendar year begins at the start of Spring in the northern hemisphere: on the midnight between the two consecutive solar noons which include the instant of the Northern spring equinox, when the sun enters the northern hemisphere. Hence, the first noon is on the last day of one calendar year and the second noon is on the first day (Nowruz) of the next year.

Month names
The first day of the calendar year is also the day of the greatest festival of the year in Iran, Afghanistan and surrounding regions, called nowruz (two morphemes: (new) and  (day), meaning "new day").

Days of the week
In the Iranian calendar, every week begins on Saturday and ends on Friday. The names of the days of the week are as follows: shambe (natively spelled "shanbeh", ), yekshambe, doshambe, seshambe, chæharshambe, panjshambe and jom'e (yek, do, se, chæhar, and panj are the Persian words for the numbers one through five). The name for Friday, jom'e, is Arabic. Jom'e is sometimes referred to by the native Persian name, adineh. In most Islamic countries, Friday is the weekly holiday.

Calculating the day of the week is easy, using an anchor date. One good such date is Sunday, 1 Farvardin 1372, which equals 21 March 1993. Assuming the 33-year cycle approximation, move back by one weekday to jump ahead by one 33-year cycle. Similarly, to jump back by one 33-year cycle, move ahead by one weekday.

As in the Gregorian calendar, dates move forward exactly one day of the week with each passing year, except if there is an intervening leap day when they move two days. The anchor date 1 Farvardin 1372 is chosen so that its 4th, 8th, ..., 32nd anniversaries come immediately after leap days, yet the anchor date itself does not immediately follow a leap day.

Seasonal error
The image below shows the difference between the Iranian calendar (using the 33-year arithmetic approximation) and the seasons. The Y axis is "days error" and the X axis is Gregorian calendar years. Each point represents a single date on a given year. The error shifts by about 1/4 day per year, and is corrected by a leap year every 4th year regularly, and one 5 year leap period to complete a 33-year cycle. One can notice a gradual shift upwards over the 500 years shown. The Gregorian calendar, introduced in 1582, is almost as accurate in the long term, but has larger swings of seasonal errors over centuries.

Relationship with the Zodiac Signs
Each month of the current Iranian calendar corresponds to the 12 signs of the zodiac in western tropical astrology. The vernal equinox or first point of aries are taken to be the beginning of the solar year.

Accuracy
It is one of the oldest calendars in the world as well as the most accurate solar calendar in use today. Since the calendar uses astronomical calculation for determining vernal equinox, it has no intrinsic error, but this makes it an observation based calendar. According to a proposal made by Ahmad Birashk, a complex mathematical pattern can be used to make the calendar a purely mathematical one without the need for astronomical observation. This proposed calendar has a great grand cycle of 2820 years in which 2137 years are normal years of 365 days and 683 years are leaps of 366 days, averaging a day-length of 365.24219852, over the 2820 years of the great grand cycle. This average is just 0.00000026 day shorter than the actual solar year of 365.24219878 days, making an accumulated error of just one day over 3.8 million years or approximately 0.022 of a second annually.



Public holidays and anniversaries
There are also a few anniversaries celebrated by Iranians, in and out of Iran alike, that are not formally endorsed by the Iranian government and not printed on calendars in Iran. These include Shab-e-Yalda, a celebration of winter solstice, which has roots in Mitraism.

Solar Hejri and Gregorian calendars
The Solar Hejri year begins about 21 March of each Gregorian year and ends about 20 March of the next year. To convert the Solar Hejri year into the equivalent Gregorian year add 621 years to the Solar Hejri year (provided the Gregorian day is 21 March or later; if earlier in the year, add 622).

Correspondence of Solar Hejri and Gregorian calendars (years beginning on 20 March are marked *, others start on 21 March: