According to the legislation passed by the Iranian Parliament in 2007, the official time of the country turns back an hour every year at midnight on September 20.
The official time of the country is also turned forward one hour every year at midnight on March 20 when the DST begins in Iran.
DST is a seasonal time change measure where clocks are set ahead of standard time with the beginning of spring, usually by one hour. As DST starts, the sun rises and sets later, on the clock, than the day before and it ends with the beginning of autumn.
In Iran the clock moves forward one hour on March 21 (the start of Iranian calendar year) and it will move back after 6 months on the first day of autumn.