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Date: Mon, 27 Oct 1997 11:52:46 -0600
From: Jonathan Lemon <jlemon@americantv.com>
To: Stephen Roome <steve@visint.co.uk>
Cc: "Jonathan M. Bresler" <jmb@FreeBSD.ORG>, freebsd-questions@FreeBSD.ORG
Subject: Re: 2000 Compliance / dates / time libs
References: <199710271629.IAA00243@hub.freebsd.org> <Pine.BSF.3.95.1000229045221.14383D-100000@dylan.visint.co.uk>
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In-Reply-To: <Pine.BSF.3.95.1000229045221.14383D-100000@dylan.visint.co.uk>; from Stephen Roome on Feb 02, 2000 at 04:57:07AM +0000
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On Feb 02, 2000 at 04:57:07AM +0000, Stephen Roome wrote:
> On Mon, 27 Oct 1997, Jonathan M. Bresler wrote:
> 
> > Stephen Roome wrote:
> > > 
> > > 
> > > I just set my clock to february 29 2000. Is this valid ?
> > 
> > 	the algorithm, as i understand it is:
> > 
> > 	if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
> > 		it is a leap year.
> > 
> > 	this is only good back till the switch from julian to gregorian
> > 	calendars.
> 
> [You stole this from k&r didn't you =), well, that's the only place I
> remember seeing this, especially in this format in C.]

Actually, the exact same logic is used in FBSD; from libc/stdtime/tzfile.h:

#define isleap(y) (((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0))

Crosschecking, Oracle also says its a leap year:

  1* select to_date('29-FEB-2000', 'DD-MON-YYYY') from dual
  ---------
  29-FEB-00

  1* select to_date('30-FEB-2000', 'DD-MON-YYYY') from dual
  ERROR:
  ORA-01839: date not valid for month specified


> I've heard (I don't trust this source though!) that there maybe an ISO
> committee for this. 

Some time/date code that I have refers to the ``ISO/R 2015-1971 and 
DIN 1355'' standards, so perhaps that may be what you mean?
--
Jonathan