Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!sun-barr!apple!motcsd!xdos!doug
From: doug@xdos.UUCP (Doug Merritt)
Newsgroups: comp.lang.c
Subject: Re: Day of week routine
Keywords: Does anybody have this?
Message-ID: <322@xdos.UUCP>
Date: 22 May 89 14:37:21 GMT
References: <234@zeek.UUCP>
Reply-To: doug@xdos.UUCP (Doug Merritt)
Distribution: usa
Organization: Hunter Systems, Mountain View CA (Silicon Valley)
Lines: 26

In article <234@zeek.UUCP> larry@zeek.UUCP (Larry Podmolik) writes:
>I need a C function that, given a date, would return what day of the
>week it fell on.  ...

Try this:
	((year * 1.25) + (day-of-the-year) + constant) modulo 7

...where day-of-the-year is obtained by adding day-of-month to sum
of days in each month prior to month-of-year, and I forget the exact
value of constant, but it's trivial to obtain by using the formula
on today's date, and seeing what adjustment is needed (it really is
a constant).

An adjustment for the switchover from the Julian to the Gregorian
calendar may be needed depending on the era and country of interest.

A variation on this formula (involving a table of constants for each
month in place of day of the year) can be used to easily *mentally*
calculate day of the week. All calculations can be done modulo 7,
which makes it easier.

The factor of 1.25 is what makes it work despite leap years.
	Doug
-- 
Doug Merritt		{pyramid,apple}!xdos!doug
Member, Crusaders for a Better Tomorrow		Professional Wildeyed Visionary