cal-persia.el disagrees with Iranian calendar in A.D. 2025

[Paul Eggert]

A time-zone database user reported a bug for the year 2025 in Iran.
This appears to be due to a disagreement between GNU Emacs's
cal-persia.el and the user's (German) source for the Persian calendar.
The problem occurs both with Emacs 21.4 and CVS head.

I enclose the user's email below.  I confirmed that

   (calendar-gregorian-from-absolute
      (calendar-absolute-from-persian '(1 1 1404)))

returns (3 20 2025), so it does appear that cal-persia disagrees with
his source, which says that 1 Farvardin 1404 corresponds to 21 March
2025.  I don't know whether this should be a code fix or an
enhancement (cal-iran.el, say?).


> From: Oscar van Vlijmen <ovv@hetnet.nl>
> Subject: Iran calculated DST dates
> Date: Wed, 30 Mar 2005 14:38:46 +0200
> 
> 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
>