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Date: Thu, 30 Aug 2018 15:33:49 -0500
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Subject: Proposal: Add tabular Islamic calendar to calendar conversion functions
From: weberk294@gmail.com (Kurt Weber)

Hi, everyone.

The calendar conversion functions currently support (via Julian Day
Numbers as an intermediary) conversion between Gregorian, Julian,
Hebrew, and French Revolutionary calendars.  Conspicuously absent is
the Islamic calendar.  A comment in ext/calendar/sdncal.h seems to
suggest that the original author(s) intended to implement at some
point, but never got around to it.

The Islamic calendar offers some complications because in most places
and communities that use it for religious purposes, the switch from
one month to the next is based on actually observing the moon in the
proper phase.  In theory the date of this change can be calculated
(and indeed the early Islamic surge in study of astronomy was
motivated by the wish to know about when they should start looking),
but tradition still demands that it be actually observed before a
change is made--the actual impact of this is that local sky or weather
conditions can sometimes interfere with the observation in a
particular jurisdiction, meaning that often it comes a day or so later
than when it otherwise would.

Consequently, what I'm proposing is the Tabular Islamic Calendar,
which was specifically created to be predictable and calculable.  It
is of limited use for religious purposes (and documentation should
probably be clear about this), but it would be useful for, e.g. people
working with historical documents from Islamic communities or groups
(In fact, this is how I came to this problem--I am working on a Ph.D.
in Russian history, and as a side project I'm developing a PHP-backed
website to manage research notes and other information; because I work
on prerevolutionary Russia I'm very familiar with the issue of needing
to convert between calendar systems (in my case, Julian and
Gregorian), and my colleagues working on Islamic history often have
similar issues.).

Tabular Islamic calendar is not ideal, but it seems like the only
realistic option for automated conversion.

As far as implementation goes, I could do it.  I've already
implemented conversion one way, before I stopped work in case there
were issues that come up in this discussion that I hadn't foreseen
(unfortunately, that happened roughly at the time the mailing list
server seems to have died, so it's been paused for a couple of weeks).
So all I'd have to do is the other direction, and it'd be ready to
test.

--Kurt Weber