People here need to keep track of three different calendars: Solar for Iranian Calendar and Lunar (Islamic) Calendars for local affairs, and Western for dealing with the outside world.
The Iranian calendars are a succession of calendars invented or used for over two millennia in Iran (Persia). One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes.
The modern Iranian calendar is now the official calendar in Iran. It begins at the midnight nearest to the instant of the vernal equinox as determined by astronomical calculations for the Iran Standard Time meridian (52.5°E or GMT+3.5h). It is, therefore, an observation-based calendar, unlike the Gregorian, which is rule-based.
The Iranian year usually begins within a day of 21 March of the Gregorian calendar. To find the corresponding year of the Gregorian calendar, add 621 or 622 (depending on the time of the year) to a solar hijri year.