Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!tank!shamash!com50!midgard!syntel!dal
From: dal@syntel.UUCP (Dale Schumacher)
Newsgroups: comp.sys.atari.st
Subject: Re: Will Your SW Make it to the year 2000?
Summary: Use magic Julian date formula to find leap years
Keywords: Julian Date
Message-ID: <061089A1311@syntel.UUCP>
Date: 10 Jul 89 14:32:24 GMT
Reply-To: dal@syntel.UUCP (Dale Schumacher)
Lines: 56
X-Member-Of: STdNET (ST Developer's Network)

[hjg@amms4.UUCP (Harry Gross) writes...]
> In article <381@amms4.UUCP> I wrote:
[several leap-year calculations and corrections deleted]
> 
> 	if (year%4 == 0 && year%100 != 0 || year%400 == 0)
> 		printf("LEAP YEAR!");
> 
> That test comes from the K & R book, from their date conversion routine
> somewhere around page 103 (113?).  I should have looked before I leapt :-)

Another approach is to use the Julian date to determine if there is a
Feb 29th in a certain year.  The following is a nice computation for
Julian date (from dLibs) which will remain valid far into the future:

long julian_date(time)
	register struct tm *time;
/*
 *	Number of days since the base date of the Julian calendar.
 */
	{
	register long c, y, m, d;

	y = time->tm_year + 1900;	/* year - 1900 */
	m = time->tm_mon + 1;		/* month, 0..11 */
	d = time->tm_mday;		/* day, 1..31 */
	if(m > 2)
		m -= 3L;
	else
		{
		m += 9L;
		y -= 1L;
		}
	c = y / 100L;
	y %= 100L;
	return(	((146097L * c) >> 2) +
		((1461L * y) >> 2) +
		(((153L * m) + 2) / 5) +
		d +
		1721119L );
	}

To determine if a given year is a leap year, calculate the Julian date of
Feb 28th and March 1st of that year and see if they are 1 or 2 days apart.
In addition, since Julian date is a linear count of days, it makes date
difference computations much easier.  Used with a modulus operation, it
also provides an easy way to determine day of the week, like this:

	t->tm_wday = (jdate + 1) % 7;	/* 0=Sunday ... 6=Saturday */

PS.  The tricky part is converting back from a Julain date to month/day/year.

\\   /  Dale Schumacher                         399 Beacon Ave.
 \\ /   (alias: Dalnefre')                      St. Paul, MN  55104-3527
  ><    ...umn-cs!midgard.mn.org!syntel!dal     United States of America
 / \\   "What is wanted is not the will to believe, but the will to find out,
/   \\  which is the exact opposite." -Bertrand Russell