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

[Ed Reingold <reingold@emr.cs.iit.edu>]

The problem is that in 2025 the equinox will be close to noon in Iran--too
close to say precisely what the calendar will be: it will be either Mar 20 or
Mar 21 on the ASTRONOMICAL calendar.  Part of the problem is that it is not
clear what location in Iran should be used, Tehran (which we chose) or 52.5 E.
The ARITHMETICAL calendar gives the Mar 20 date.  What will the TRUE date be?
It's anybody's guess, but PROBABLY Mar 21.

I don't see that any sensible change is possible at this time to either Emacs
(which does not contain the mechanisms in place to implement the astronomical
calendar) nor in the time-zone data base.


> 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
> >