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From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac
Subject: Re: Week DAYS (was Re: why 1904?  ms excel)
Message-ID: <22033@cup.portal.com>
Date: 10 Sep 89 11:02:29 GMT
References: <457a7553.1285f@maize.engin.umich.edu>
  <8909061947.AA00416@vs04csc.UMD.EDU> <972@mrsvr.UUCP>
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Organization: The Portal System (TM)
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The formula is not too hard to work out.  If you want to, however, you
will find it much easier if you pretend that March 1 is the start of
the year.

If you want to find day-of-week from a given date in this century, there
is a simple method Martin Gardner gave once that is easy to do in your
head.  You need to memorize the following 12 numbers:

	1 4 4 0 2 5 0 3 6 1 4 6

Let's say you want the day of the week for M/D/19YY.  You first divide
YY by 12, given, say, A with a remainder of B.  Divide B by 4.
Add these numbers together.  In other words,

	YY/12 + YY%12 + (YY%12)/4

Let's call this N.

Now use the month, M, to look up a number in that table given above.
Add this to N.  Then add D.  Reduce this mod 7.  This gives you the
day of the week, with 0 == Saturday, etc.

For example, today is 9/10/1989.

	89/12 = 7
	89%12 = 5
 	 5/4  = 1
       ----------
	   N  = 13, which we can reduce mod 7 to 6.  You might as well
		remember this number, since it will be valid all year.

	Number for September ( 9th number in 1 4 4 0 2 5 0 3 6 1 4 6 )
	is 6.

	Thus, today is 6  + 6 + 10 = 22.

	Mod 7, this is 1, or Sunday.  Hey, it worked!

Another example, 12/7/1941.  41/12 = 3, 41%12 = 5, 5/4 = 1.
Number for December = 6.  Day of week is 9+6+7 = 1 mod 7
which is Sunday.

If the year is a leap year, modify the month table as follows for
that year:
	0 3 4 0 2 5 0 3 6 1 4 6

						Tim Smith