[tz] Timezones Morocco 2013 incorrect

[Paul Eggert]

On 03/06/2013 09:13 AM, Patrice Scattolin wrote:

> Since we have no idea of emacs' algorithm

We have the source code, so we can get an idea.
The code can be found here:

http://bzr.savannah.gnu.org/lh/emacs/trunk/annotate/head:/lisp/calendar/cal-islam.el

The function calendar-islamic-to-absolute is the key part.
I read it, and found that GNU Emacs uses the most-popular
arithmetical approximation to the Islamic calendar.  By "most-popular"
I mean that it's the approximation used by the majority of the Moslem world,
according to page 84 of Dershowitz N, Reingold EM, Calendrical Calculations,
3rd ed. (2008). Cambridge University Press, ISBN 978-0-521-70238-6.

Dershowitz and Reingold mention that there are other arithmetical
approximations.  In particular, some Microsoft products
use a variant approximation.  So it appears that no matter which
approximation we'll use, we're bound to disagree with somebody's
else's prediction.

I'll soon circulate a proposed patch to the 'africa' file
that makes predictions based on the GNU Emacs approximation.