Iranian Calendar or Persian Calendar (Persian: گاهشماری ایرانی) Is a calendar that has been in use in Greater Iran for almost two millennia. At different times in history, this calendar has been reformed to facilitate climate , religious and administrative work. The modern Iranian calendar or solar hijri is the national calendar of Iran and Afghanistan. According to this calendar, the year begins on the day of the equinox in Iranian time. This day of the beginning of the year is calculated in a more accurate way than the Gregorian calendar. This is because this calendar relies on astronomical calculations rather than mathematical rules to determine the actual time of the equinox. In English, the abbreviated form of AP , the Latin word Anno Persico , is used to mark the year of this calendar. In the Iranian calendar, the year usually begins on or after March 21, one or two days later. The year can be determined by adding 621 or 622 to this year.