Category : C Source Code
Archive   : WEEKDAY.ZIP
Filename : WEEKDAY.C

/*----------------------------------------------------------------------
*+
* Compute_Day
* Compute Day of Week
*
* Usage
*
* int
* Compute_Day(date, month, year)
* int date ;
* int month ;
* int year ;
*
* Parameters
*
* date Date of Month (1 - 31)
* month Month of Year (1 - 12)
* year Year (e.g., 1989)
*
* Description
*
* Compute_Day() computes the day of the week for the given date
* using the Zeller Congruence. It returns a positive value from
* 0 to 6 for the day of the week
*
* Notes
*
* The Zeller Congruence maps Saturday as day 0. Most applications
* treat Sunday as day 0. The parameter ZELLER_OFFSET is used to
* convert the day of the week from Zeller to local.
*
*-
*/

int
Compute_Day(date, month, year)
int date ;
int month ;
int year ;

{
int day ;
int yr ;
int mn ;
int n1 ;

int n2 ;

/* Offset from Zeller to local */
/* --------------------------- */
#define ZELLER_OFFSET -1

yr = year ;
mn = month ;

/* January or February? */
/* -------------------- */
if (mn < 3)
{
/* Yes, make these part of last year */
/* --------------------------------- */
mn += 12 ;
yr -= 1 ;
}

n1 = (26 * (mn + 1)) / 10 ;
n2 = (int) ((125 * (long) yr) / 100) ;

day = ((date + n1 + n2 - (yr / 100) + (yr / 400) + ZELLER_OFFSET) % 7) ;
return day ;
}