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

[Paul Eggert]

Ed Reingold <reingold@emr.cs.iit.edu> writes:

> For the TZ data base, what is the point of making one assumption over the 
> other?  Neither will be known right or wrong for another 20 years.

It's mostly a matter of documenting the projected daylight-saving
transitions.  Currently we project through 2038 (when 32-bit time_t
values roll over).  We are adding support for 64-bit time_t values,
though, and at some point we'll probably start projecting a bit
further (50 years, say? we haven't decided).  The cutoff is somewhat
arbitrary, but going a few decades into the future helps give novices
a better feel for the complexity of the projections.

By the way, in researching this I found the following reference useful:

  M. Heydari-Malayeri (Paris Observatory),
  A concise review of the Iranian calendar (2005-02-15),
  <http://wwwusr.obspm.fr/~heydari/divers/ir-cal-eng.html>

It mentions the March 20, 2025 discrepancy, and it has some
interesting and not-altogether-positive things to say about the method
used in GNU Emacs.  I hadn't realized how controversial this area is.


> For Emacs, it's irrelevant because it does not contain the Persian
> astronomical calendar

Thanks for clarifying this.  Would it be appropriate to make the
following change to the GNU Emacs user documentation, if only to help
forestall future bug reports in this area?

2005-03-31  Paul Eggert  <eggert@cs.ucla.edu>

        * calendar.texi (Calendar Systems): Mention that the Persian
        calendar implemented is the arithmetical calendar of Birashk.

--- calendar.texi.~1.33.~       2005-03-28 16:30:06 -0500
+++ calendar.texi       2005-03-31 01:46:45 -0500
@@ -691,6 +691,12 @@ Their calendar consists of twelve months
 days, the next five have 30 days, and the last has 29 in ordinary years
 and 30 in leap years.  Leap years occur in a complicated pattern every
 four or five years.
+The calendar implemented here is the arithmetical Persian calendar
+championed by Birashk, based on a 2,820-year cycle.  It differs from
+the astronomical Persian calendar, which is based on astronomical
+events.  As of this writing the first future discrepancy is projected
+to occur on March 20, 2025.  It is currently not clear what the
+official calendar of Iran will be that far into the future.
 
 @cindex Chinese calendar
   The Chinese calendar is a complicated system of lunar months arranged