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From: simpson@aplcen.apl.jhu.edu (Simpson David Grant)
Newsgroups: comp.lang.pascal
Subject: Re: Name of day from date & year?
Keywords: dayname date year day
Message-ID: <1991Feb18.161533.21099@aplcen.apl.jhu.edu>
Date: 18 Feb 91 16:15:33 GMT
References: <1991Feb15.121713.10075@newcastle.ac.uk>
Reply-To: simpson@aplcen (Simpson David Grant)
Organization: Johns Hopkins University
Lines: 29

In article <1991Feb15.121713.10075@newcastle.ac.uk> Boye.Tranum@newcastle.ac.uk (B. Tranum) writes:
>I need to find out the name of the day (Monday, Tuesday etc.) from the date
>and the year. For example, which day was 10-02-91. - It was a Sunday.
>
>Does anybody have a nice routine, in Pascal or in English, to do this. I would 
>be very pleased if you could help me.
>


There is an article in the latest issue of the Journal of Recreational
Mathematics that has a routine for doing this.  Given a month m,
day d, and year y in the Gregorian calendar, the day of week D (where
0=Sunday, 1=Monday, ..., 6=Saturday) can be found from

D = ((23*m/9)+d+4+y+(z/4)-(z/100)+(z/400)-2(if m>=3)) mod 7

where
   z = y-1 if m<3
   z = y   otherwise
(See "The Ultimate Perpetual Calendar" by Keith & Craver, J. Rec.
Math., v22 n4 p280).  They include a (if you'll excuse this) C
function:

dayofweek (y,m,d)
{
  return ((23*m/9+d+4+(m<3?y--:y-2)+y/4-y/100+y/400)%7);
}

                                    David Simpson