Solar Hijri calendar
The Solar Hijri calendar (Persian: گاهشماری هجری خورشیدی, romanized: Gâhšomâri-ye Hejri-ye Xoršidi; Pashto: لمريز لېږدیز کلیز, romanized: lamrez legdez kalhandara) is a solar calendar and one of the various ancient Iranian calendars. It begins on the March equinox as determined by 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 of both Iran and Afghanistan, and is sometimes also called the Shamsi Hijri calendar, and abbreviated as SH and, sometimes, HS.
The Solar Hijri 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 has the same epoch (start date) as the Lunar Hijri calendar used by the majority of Muslims (known in the West as the Islamic calendar): the Hijrah, the journey of the Islamic prophet Muhammad and his followers from Mecca to Medina in the year 622. Unlike the latter, its years are solar years rather than lunar years.
Each of the twelve months corresponds with a zodiac sign; their names are the same as ancient Zoroastrian names from the Zoroastrian calendar – in Afghanistan on the other hand, the names of the zodiacal signs are used instead. The first six months have 31 days, the next five have 30 days, and the last month has 29 days in usual 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 and Afghanistan.