Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!sdrc!scjones
From: scjones@sdrc.UUCP (Larry Jones)
Newsgroups: comp.sys.ibm.pc
Subject: Re: I need a date :) please help
Message-ID: <1268@sdrc.UUCP>
Date: 8 Apr 90 14:27:33 GMT
References: <19500060@uxh.cso.uiuc.edu>
Organization: SDRC, Cincinnati
Lines: 31

In article <19500060@uxh.cso.uiuc.edu>, mead@uxh.cso.uiuc.edu writes:
> QUERY:  Does anyone know of *any* source for an intelligent algorythm
> for getting the day ('Monday, Tuesday, ...') from the date (4/3/90).
> My PC does it like lightning each time I issue the 'DATE' command, and
> if IBM can do it, anyone can - right? :)  Textbooks or whatever are
> fine, but please don't direct me to existing software unless it's PD
> source code (in BASIC, C, Pascal, or FORTRAN).  Thanks.

The traditional algorithm is (in C):

	weekday = (year + year/4 + startday[month-1] + day) % 7;
	if (year%4 == 0  &&  month < 3) --weekday;

year is either the full year or the last two digits, month is the
month (1 - 12), and day is the day of the month (1 - 31).

startday is a table of the day of the week each month starts on
(in a non-leap year).  Start by looking at a calendar and using
the day of the week each month starts on (0=Sunday, 7=Saturday).
Then, try an example and adjust all the table entries to get the
correct result.  (For example, if the date you try is a Tuesday
(2) and the equation gives you Friday (6), then you need to
subtract 4 (6-2) from each entry in the table.  If the result is
negative, just add 7.)
----
Larry Jones                         UUCP: uunet!sdrc!scjones
SDRC                                      scjones@SDRC.UU.NET
2000 Eastman Dr.                    BIX:  ltl
Milford, OH  45150-2789             AT&T: (513) 576-2070
"You know how Einstein got bad grades as a kid?  Well MINE are even WORSE!"
-Calvin