Solar Hijri calendar
The Solar Hijri calendar (Persian: گاهشماری هجری خورشیدی, romanized: gāhshomāri-ye hejri-ye khorshidi; Pashto: لمريز لېږدیز کلیز), also called the Iranian Hijri calendar or Shamsi Hijri calendar, and abbreviated as SH and, sometimes, HS, is the official calendar of Iran and Afghanistan. It begins on the March equinox (Nowruz) 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.
Its determination of the start of each year is astronomically accurate year-to-year as opposed to the more fixed Gregorian or Common Era calendar which, averaged out, has the same year length, achieving the same accuracy (a more simply patterned calendar of 365 days for three consecutive years plus an extra day in the next year, save for exceptions to the latter in three out of every four centuries). The start of the year and its number of days remain fixed to one of the two equinoxes, the astronomically important days when day and night each have the same duration. It results in less variability of all celestial bodies when comparing a specific calendar date from one year to others.
Each of the twelve months corresponds with a zodiac sign. 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 Iranian New Year's Day always falls on the March equinox.