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Newsgroups: comp.sys.apple2.programmer
Subject: Days of week & No Slot Clock
From: tombo@mail.on-line.co.uk (Richard King)
Distribution: world
Message-ID: <CpC5o5.2nr@mail.on-line.co.uk>
Sender: tombo@mail.on-line.co.uk (Richard King)
Organization: On-line Entertainment
Date: Thu, 5 May 1994 15:28:53 GMT
Lines: 25

The formula for calculating the day of the week from the year, month and
day is known as Zeller's Congruence, and is performed by the following
ANSI-C subroutine.

int     zeller(int day, int month, int year)    /*  determine day of week
  */
{
    int     y1,
            y2;
 
    if (month < 3)
    {
        month += 10;
        year -= 1;
    }
    else
        month -= 2;
    y1 = year / 100;
    y2 = year % 100;
    return ((day + ((26 * month - 1) / 10) + y2 + y2 / 4 + y1 / 4 - 2 *
y1 + 49) % 7);
}


Hope this solves any questions.