Re: Personal Calendar Server Volunteer? (fwd)

[rms39 columbia edu (Russell Steinthal)]

Ali,

The idea of integrating lunar calendar support into 
gnomecal/Evolution is interesting, and one I've given some thought to 
in the past, but in a slightly different context.  One of the 
projects which has sat in a perpetually unfinished (and unworked on, 
frankly) state on my computer has been a Hebrew Calendar program, 
which would also be lunar-based; I've never given any *serious* 
thought to integrating it into gnomecal, however, beyond possibly a 
CORBA interface to mark Jewish holidays, for example, in my gnomecal.

That being said, I am reasonably familiar with the issues involved in 
such a calendar implementation, so here are my thoughts:

On Thu, 03 Feb 2000 16:34:45 +0300, Ali Abdin wrote:

>Anyway, I was also thinking a bit about this and realized that the 
>'native' date/time format on Linux/Unix is Gregorian based. Would it 
be
>'feasible' to implement a low-level Linux/Unix solution first or 
should
>the Lunar Calendar be based on some sort of 'conversion' from the 
date in
>Linux/Unix. Hmmm, come to think of it - doesn't the Linux/Unix date 
just
>return a 'seconds elapsed' since 1979 or something like that?

Building a kernel-level infrastructure is simply not worth the 
trouble, IMHO.  The PC clock counts (fractions of) seconds, which is 
what time_t stores (seconds since the epoch).  The infrastructure is 
Gregorian to the extent that libc has code for time_t to Gregorian 
conversions (localtime, etc.).  If you thought there was a 
general-purpose need for Islamic time calculations, you could, I 
suppose submit patches to glibc, but I think that for most purposes a 
separate library would be fine.  (I wonder if the glibc locale 
support is general-purpose enough to handle a different calendar 
system, rather than simply a different format?  Just curious...)

A related problem, though, is that there is no such thing as a single 
"lunar" calendar.  The Islamic calendar, for example, differs from 
the Hebrew calendar. (The Hebrew calendar, for example, is actually 
lunar/solar, in the sense that there's an intricate system of leap 
months used to keep the lunar months from floating through the 
seasons.)  So any solution you build should be sufficiently modular 
to allow any number of different calendar systems.

>Since the major effort is all going into evolution, I wanted to 
'explore'
>that possibility. Specifically, I was interested on what is required 
for
>lunar calendar support for Evolution. I come from a culture (Arab) 
that
>uses the Lunar Calendar, so I am pretty experienced with it (there 
are
>some Lunar Calendar quirks since one month can be either 29/30 days 
>depending on Moon Sighting). I believe this is just in the "Islamic" 
>calendar (which is lunar based). Enough rambling.

What would be required?  Offhand, here's what I came up with:

1. A library to handle conversions from an arbitrary time_t to a 
"lunar" date/time.  (Subject to the vagaries of the multiple calendar 
point I mentioned above.)

2. Modified versions of the calendar widgets used in gnomecal which 
don't contain Gregorian assumptions like the number of months, the 
number of days per month, month names, etc.

3. If you want to make this a native format, you need code to handle 
loading and saving.  Interestingly, the iCalendar specification 
implicitly supports multiple calendar systems- that is, there's a 
CALSCALE property which can theoretically designate the calendar 
format in use for that file.  As the RFC says, however:

   This memo is based on the Gregorian calendar scale. The
   Gregorian calendar scale is assumed if this property is not 
specified
   in the iCalendar object. It is expected that other calendar scales
   will be defined in other specifications or by future versions of 
this
   memo.

In theory, at least, all the rest of gnomecal should function as 
before, since internally, we use time_t + TIMEZONE (or at least we 
soon will; right now, timezone support is a bit shaky).

Figuring out how to handle the moon-sighting issue is also a hairy problem, but I'll leave that to you as an exercise. :)  (The Hebrew calendar, at least, is algorithmically-generated.)

-Russell