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Re: Calendars (was: Re: leap second)



> > Run "cal 9 1752" and tell me that.
[..]
> A more serious problem is that the current implementation doesn't allow  
> for non-Christian date systems, of which there are several in active use.  
> I'd expect that to be a problem for people in both parts of Jerusalem, for  
> example.
> 
> Does anybody know enough about those other systems to tell if the general  
> design would at least work - that is, dates are year/month/day tuples?

Well, about the Muslim calander: year/month/day works for representing
dates, the only problem is that officially you can only tell the
dates in the past, not in the future: the beginning of the next month
is signaled by the moon, and although the position of the moon can be
preditect quite acurately nowadays, it that couldn't be done in
Mohammeds time. So, the next month only starts when _people_see_
the new moon -- and that's impossible to predict reliably.
(This is a problem with ramadan (the nineth month): they never
know exactly when it starts/ends).

> > Posix time includes leap-year-days, but does not include the finer
> > resolution of leap-seconds. 21 leap-seconds (number 22 is coming up)
> > have been added since New Years Day 1970 to keep clock time in synch
> > with astronomical time.
> 
> Actually, it probably was a bad idea to use "leap" for both. Leap days are  
> fixed by calendar design. Leap seconds are inserted or deleted (both are  
> possible) after comparing the atomic clocks to astronomical observations,  
> with no predictability at all. Two very different animals.

well, depends on how you see it. The before 1752, century turns were
still all leap years. Now, we know the length of a year/day better, and
only 1 in for of those turn-of-century years are leap years. Maybe that
will change again. And about the seconds: we (currently, prossibly always)
simply cannot calulate the length of a day accurately enough to know
well in advance when to insert them. But I'd say the two animals are
at least related, if not mother and daughter.


-- 
joost witteveen, joostje@debian.org
#!/usr/bin/perl -sp0777i<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<j]dsj
$/=unpack('H*',$_);$_=`echo 16dio\U$k"SK$/SM$n\EsN0p[lN*1
lK[d2%Sa2/d0$^Ixp"|dc`;s/\W//g;$_=pack('H*',/((..)*)$/)
#what's this? see http://www.dcs.ex.ac.uk/~aba/rsa/


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