The Solar Hijri calendar is a solar calendar and one of the various Iranian calendars. It begins on the March equinox as determined by the astronomical calculation for the Iran Standard Time meridian (52.5°E, UTC+03:30) and has years of 365 or 366 days. It is the modern principal calendar in Iran and is sometimes also called the Shamsi calendar and Khorshidi calendar. It is abbreviated as SH, HS or, by analogy with AH, AHSh.
The ancient Iranian Solar calendar 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 the vernal equinox, it has no intrinsic error. It is older than the Lunar Hijri calendar used by the majority of Muslims (known in the West as the Islamic calendar); though they both count from the Hijrah, the journey of the Islamic prophet Muhammad and his followers from Mecca to Medina in the year 622, one uses solar years and the other lunar years.
Each of the twelve months corresponds with a zodiac sign, and in Afghanistan the names of the zodiacal signs were used for the months; elsewhere the month names are the same as in the Zoroastrian calendar. The first six months have 31 days, the next five have 30 days, and the last month has 29 days in common years but 30 days in leap years.
The ancient Iranian New Year's Day, which is called Nowruz, always falls on the March equinox. While Nowruz is celebrated by communities in a wide range of countries from the Balkans to Mongolia, the Solar Hijri calendar itself remains only in official use in Iran.
Structure
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 (). Jom'eh is sometimes referred to by the native Persian name, ādineh (). In some Islamic countries, like Iran and Afghanistan, Friday is the weekly holiday.