I checked the DST on/off dates for Iran. It appears that the dates were calculated with the arithmetic 'Persian' calendar in mind. Since 1925 an astronomical calendar is in use, so I read. There is one day difference between these two calendar systems in the Gregorian year 2025, so there is one error in the list of DST rules for Iran if indeed the astronomical calendar system is used. The astronomical Iranian calendar has 1 Farvardin 1404 on 21 March 2025 Gregorian, so DST on is per 22 March 2025 0:00, and 30 Shahrivar 1404 is on 21 September 2025 Gregorian, so DST off is per 22 September 2025 Gregorian. The vernal equinox in 2025 Gregorian is at around 09:01 UTC on March 20, so around 12:27 Tehran 'solar' time. This is past 12:00, hence 1 Farvardin is on March 21. A long article about the Iranian calendar (in German): Iranische Zeitrechnungen, Nikolaus A. Bär, 2004. See the chapter "Der neuiranische Kalender", with a French translation of the 1925 law. http://www.nabkal.de/irankal.html I checked the Iranian calendar dates with the Java application by Reingold/Dershowitz, version 2.1. http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html And the mentioned vernal equinox date/time with 2 different astronomical programs. Oscar van Vlijmen 2005-03-30