The Solar Hijri calendar is a solar calendar, meaning that its time reckoning is based on the Earth's movements around the Sun. Unlike the Gregorian calendar, which follows a set of predetermined rules to stay in sync with the solar year, the Solar Hijri calendar is based on astronomical observations. The year begins at midnight closest to the vernal equinox in Iran – specifically at the Iran Standard Time meridian at longitude 52.5° east, which runs about 250 miles east of Tehran. The first day of the new year is called Nowruz, and it is celebrated around the world by Iranian people.
Tying the Solar Hijri calendar so closely to the astronomical seasons makes it much more accurate than the Gregorian calendar, which, even in its modern form, deviates from the solar year by 1 day in 3236 years.
A year in the Solar Hijri calendar is divided into 12 months of varying lengths. The first 6 months have 31 days, and months 7 through 11 have 30 days. The last month, Esfand, has 29 days in a common year and 30 days in a leap year.
The Solar Hijri year count starts with the Islamic prophet Mohammed's migration (Hegira or Hijrah) to Medina in 622 CE. Although the Solar Hijri calendar shares this start date with the Islamic calendar (Hijri calendar), the calendar systems are not related otherwise.
The Hijri calendar is a lunar calendar. Because of this, the year counts between the Solar Hijri calendar and the Hijri calendar differ substantially. For example, January 1, 2016 fell into year 1394 in the Solar Hijri calendar, which corresponds to year 1437 in the Hijri calendar.
Like in the Gregorian calendar, a common year in the Solar Hijri calendar has 365 days while a leap year has 366 days. However, because the Solar Hijri calendar is an observational calendar, there are no mathematical rules to determine leap years. Instead, it is the number of days between two vernal equinoxes that determines if Esfand has 29 or 30 days.
The Solar Hijri calendar has been Iran's official calendar since 1925. In Afghanistan, it was introduced in 1957. The earliest forms of Iranian time reckoning date back as far as the second millennium BCE. A number of different calendar systems were used in Persia through the centuries, including the Zoroastrian calendar and the Islamic calendar. The first version of the modern Solar Hijri calendar, the Jalali calendar, was developed in the 11th century by a group of astronomers including the Persian scientist Omar Khayyam.