Pascal Calendar Program

Date: 02/12/2002 at 15:20:24
From: Danilo Tiago
Subject: Pascal gregorian calendar program

My Programing teacher gave me a holiday homework: make a program in 
Pascal that with a date of our choice give us the day of the week.
Can you help me with that?

Thanks for the attention.

Date: 02/18/2002 at 11:06:30
From: Doctor Fenton
Subject: Re: Pascal gregorian calendar program

Hi Danilo,

Thanks for writing to Dr. Math. The best way I know to make such a
computation is to use the Julian date system. I'll give you the 
algorithm, and it should be straightforward to program.

Determining the Day of the Week from the Calendar Date:

Step 1: Determine the Julian Date (JD) of the calendar date. (The 
Julian date is the number of days that have elapsed since noon, 
January 1, 4713 B.C. Since it is primarily used by astronomers, it 
begins at noon [so that the date doesn't change while they are looking 
through their telescopes], and the Julian date of the beginning of a 
day at midnight will always be a half-integer.)

Given the year Y, month M, and day D, if M = 1 or 2, then let 
y = Y-1 and m = M+12 (this considers January and February as the 
13th and 14th months of the previous year).  Otherwise, let y = Y and 
m = M.

In the Gregorian calendar (after Oct. 15, 1582), let A = Int(y/100), 
and B = 2 - A + Int(A/4).

In the Julian calendar (before Oct. 4, 1582), take B = 0
[Note: many countries didn't adopt the Julian calendar until much 
later, so for a historical date, make sure you know which calendar was 
being used.]

Then compute the Julian Date

  JD = Int(365.25*y)+Int(30.6001*(m+1))+D+B+1720994.5 .

Examples:

1. January 3, 1985.

Since M=1, we take y=1984 and m=13.  
  A = Int(1984/100) 
    = Int(19.84)
    = 19

and
 
  B = 2 - 19 + Int(19/4)
    = 2 - 19 + 4
    = -13

Then

 JD= Int(365.25*1984)+Int(30.6001*14)+3-13+1720994.5
   = Int(724656)+Int(428.4014)+3-13+1720994.5
   = 724656 + 428 + 3 - 13 + 1720994.5
   = 2446068.5

2. December 3 1993 is JD 2449324.5.

Step 2:  To find the day of the week, add 1.5 to the Julian date 
calculated in step 1. Then find the REMAINDER W when (JD+1.5) is 
divided by 7. For example, for January 3, 1985, when we divide 2446070 
by 7, we get a quotient of  349438 and a remainder of 4:

  JD + 1.5 = 7 * 349438 + 4

If the remainder is      the day is
--------------------     ------------
     0                    Sunday
     1                    Monday
     2                    Tuesday
     3                    Wednesday
     4                    Thursday
     5                    Friday
     6                    Saturday

so January 3, 1985 was a Thursday.

December 3, 1993 was JD 2449324.5, and JD+1.5 divided by 7 leaves a 
remainder of 5, so the day is a Friday.

I hope this helps.  Please write again if you have any questions.

- Doctor Fenton, The Math Forum
  http://mathforum.org/dr.math/