Detailed Description

Solar Hijri Calendar System

The Solar Hijri calendar, also called the Solar Hejri calendar, Shamsi Hijri calendar or Jalali calendar, is the official calendar of Iran and Afghanistan. It begins on the vernal equinox (Nowruz) as determined by astronomical calculation for the Iran Standard Time meridian (52.5°E or GMT+3.5h). This determination of starting moment is more accurate than the Gregorian calendar for predicting the date of the vernal equinox, because it uses astronomical observations rather than mathematical rules.

Calendar Organization

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 New Year's Day always falls on the March equinox.

Leap Year Rules

The Solar Hijri calendar produces a five-year leap year interval after about every seven four-year leap year intervals. It usually follows a 33-year cycle with occasional interruptions by single 29-year or 37-year subcycles. The reason for this behavior is that it tracks the observed vernal equinox. By contrast, some less accurate predictive algorithms are in use based on confusion between the average tropical year (365.2422 days, approximated with near 128-year cycles or 2820-year great cycles) and the mean interval between spring equinoxes (365.2424 days, approximated with a near 33-year cycle).

Source: Wikipedia page on Solar Hijri Calendar