Epochal date

The calendar's epoch (first year) corresponds to the Hijrah in 622 CE, which is the same as the epoch of the Lunar Hijri calendar but as it is a solar calendar, the two calendars' year numbers does not coincide with each other and are slowly drifting apart, being about 43 years apart as of 2023.

Days per month

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 in common years or 30 days in leap years. This is a simplification of the Jalali calendar, in which the commencement of the month is tied to the sun's passage from one zodiacal sign to the next. The sun is travelling fastest through the signs in early January (Dey) and slowest in early July (Tir). The current time between the March and September equinoxes is about 186 days and 10 hours, the opposite duration about 178 days, 20 hours, due to the eccentricity of Earth's orbit. (These times will change slowly due to precession of the Earth's apsides, becoming inverted after around 11 500 years.)

Leap years

The Iranian Solar calendar produces a five-year leap year interval after about every seven four-year leap year intervals. It usually follows a 33-year subcycle with occasional interruptions by a single 29-year subcycle. The reason for this behaviour is (as explained above) that it tracks the observed vernal equinox.

Some predictive algorithms had been suggested, but were inaccurate due to confusion between the average tropical year (365.2422 days) and the mean interval between spring equinoxes (365.2424 days). These algorithms are not generally used (see Accuracy).

New Year's Day

The Ancient Iranian Solar calendar year begins at the start of spring in the Northern Hemisphere: on the midnight in the interval between the two consecutive solar noons that includes the instant of the March equinox. Hence, the first mid-day is on the last day of one calendar year, and the second mid-day is on the first day (Nowruz) of the next year.

Months

The first day of the calendar year, Nowruz ("New Day"), is the greatest festival of the year in Iran, Afghanistan, and some surrounding historically Persian-influenced regions. The celebration is filled with many festivities and runs a course of 13 days, the last day of which is called siz-dah bedar ("13 to outdoor").

The Dari (Afghan Persian) month names are the signs of Zodiac. They were used in Iran in the early 20th century when the solar calendar was being used.

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: shanbeh, yekshanbeh, doshanbeh, seshanbeh, chahārshanbeh, panjshanbeh and jom'eh (yek, do, se, chahār, and panj are the Persian words for the numbers one to five). The name for Friday, jom'eh, comes from Arabic (جمعه). Jom'eh is sometimes referred to by the native Persian name, ādineh [ɒːdiːne] (آدینه). In some Islamic countries, like Iran and Afghanistan, 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.

Iran

A Persian-language contract published in Tehran on 14 April 1910, which used Lunar Hijri calendar

On 21 February 1911, the second Iranian parliament adopted as the official calendar of Iran the Jalali sidereal 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.^[1] 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" ever so. It also fixed the number of days in each month, which previously varied by year with the sidereal zodiac. It revived the ancient Persian names, which are still used. It also set the epoch to the Hijrah. It also deprecated the 12-year cycles of the Chinese-Uighur calendar, which were not officially sanctioned but were commonly used.

Earlier starting year (500 BC discovered and from 1975–1979 it was reset to its original date of 500 BC)

In 1975,^[1] Iranian scholars discovered that this solar calendar was in practice as early as the Achaemenid Era. Therefore, the Pahlavi Government under the orders of Shah Mohammad Reza Pahlavi changed the origin of the calendar to the beginning of Cyrus the Great's reign as its first year, rather than the Hijrah of Muhammad. Overnight, the year changed from 1354 to 2534. The change lasted until the Iranian revolution in 1979, at which time the Iranian government reverted the epoch to the Hijrah.^[9]

Afghanistan

Afghanistan legally adopted the official Jalali calendar in 1922^[1] but with different month names. Afghanistan uses Arabic names of the zodiacal signs; for example, the Saur Revolution in 1978 took place in the second month of the Solar Hijri calendar (Persian Ordibehesht; Saur is named after Taurus). The Solar Hijri calendar has been until recently the official calendar of the government of Afghanistan,^[10] and all national holidays and administrative issues were fixed according to the Solar Hijri calendar.

However, the Taliban have imposed the lunar Hijri calendar in Afghanistan during both periods of their rule. Under the Taliban's first rule from 1996 to 2001, the lunar Hijri calendar was imposed, thus changing the year overnight from 1375 to 1417.^[11] With effect from 1 Muharram 1444 AH (8 Mordad 1401 SH; 30 July 2022 CE) (the Islamic New Year of the lunar Hijri calendar), the Taliban have once again imposed the lunar calendar. Thus the year number once again leaped forward, this time from 1401 to 1444. ^[12]

Tajikistan

Tajikistan does not use the Solar Hijri calendar and never did so, despite being part of the Persian-speaking world. Although the country does celebrate Nowruz, the official New Year's Day in Tajikistan is 1 January in the Gregorian calendar,^[13] which is also the case in other non-Persian speaking Iranian or Turkic communities ranging from Eastern Europe to Western China. The name of Tajikistan's capital, Dushanbe, is taken from the Solar Hijri calendar and translates to "Monday" in Persian.^[14]

Birashk leap year algorithm

Iranian mathematician Ahmad Birashk (1907–2002) proposed an alternative means of determining leap years. Birashk's book came out in 1993, and his algorithm was based on the same apparently erroneous presumptions as used by Zabih Behruz in his book from 1952.^[17] Birashk's technique avoids the need to determine the moment of the astronomical equinox, replacing it with a very complex leap year structure. Years are grouped into cycles which begin with four normal years, after which every fourth subsequent year in the cycle is a leap year. Cycles are grouped into grand cycles of either 128 years (composed of cycles of 29, 33, 33, and 33 years) or 132 years, containing cycles of 29, 33, 33, and 37 years. A great grand cycle is composed of 21 consecutive 128-year grand cycles and a final 132 grand cycle, for a total of 2820 years. The pattern of normal and leap years which began in 1925, will not repeat until the year 4745.

The accuracy of the system proposed by Birashk and other recent authors, such as Zabih Behruz, has been thoroughly refuted and shown to be less precise than the traditional 33-year cycle.^[17]

Each 2820-year great grand cycle proposed by Birashk contains 2137 normal years of 365 days and 683 leap years of 366 days, with the average year length over the great grand cycle of 365.24219852. This average is just 0.00000026 (2.6×10⁻⁷) of a day shorter than Newcomb's value for the mean tropical year of 365.24219878 days, but differs considerably more from the mean vernal equinox year of 365.242362 days, which means that the new year, intended to fall on the vernal equinox, would drift by half a day over the course of a cycle.^[17]