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From: azarian@hpcc01.HP.COM (Randy Azarian)
Newsgroups: comp.lang.pascal
Subject: Re: Day-of-week algorithm, please!
Message-ID: <1780001@hpcc01.HP.COM>
Date: 9 Apr 89 19:55:08 GMT
References: <19040@adm.BRL.MIL>
Organization: HP Corporate Computing Center
Lines: 33

FUNCTION calcnumdays (month,day,year : integer) : word;

(* this function calculates the total number of days elapsed between 
   parameters passed , month, day, and year, and january 1, 1901.  this
   function is used to the day of the week, by knowing that january 1,
   1901 was a tuesday.  modulus division can then determine the day of
   the week by calculating something to the effect as follows:

   IF THE TOTAL NUMBER OF DAYS MODULUS 7 EQQUALS 0 THEN IT IS A TUESDAY.

   the rest of the days of the week can be calculated relative to the above
   statement.

*)

(* daysinmonth is a function you can write yourself that returns,
   obviously, the number of days in the month.
   i.e   x := daysinmonth(2,88);  x is set to 29

   if the month is not february, then year can be anything
*)

var monthctr, total : integer;

begin
  total := day -1;
  for monthctr := 1 to month-1 do
    total := total + daysinmonth(monthctr,year);

  total := total + 365 * (year - 1);
  total := total + + (year - 1) div 4;
  calcnumdays := total;
end;