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From: mccaugh@s.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: Date-Routines for C/UNIX ? HELP
Message-ID: <207600039@s.cs.uiuc.edu>
Date: 11 Sep 89 23:53:00 GMT
References: <298@nixsin.UUCP>
Lines: 64
Nf-ID: #R:nixsin.UUCP:298:s.cs.uiuc.edu:207600039:000:2153
Nf-From: s.cs.uiuc.edu!mccaugh    Sep 11 18:53:00 1989


 I tried mailing to Mathias, but no luck...anyway, here (at least) is the
 C code for day-of-week given the date as MMDDYYYY:

#include <stdio.h>
#include <time.h>
#include <ctype.h>

 void  weekday();

 static char *names[] = {"Sunday", "Monday", "Tuesday", "Wednesday",
                                  "Thursday", "Friday", "Saturday"};

 main (argc,argv)
  int  argc;
 char *argv[];
 {
  if(argc!=2)
    fprintf(stderr, "usage:  weekday  mmddyyyy\n");
  else weekday(argv[1]);
 }


 void weekday(date)   /* given:  date = "mmddyyyy" */
        char *date;   /* print:  day of the week   */

 {
       char *dp, month[3], day[3], year[5];
        int  mm, dd, yyyy, reg_yrs, lp_yrs,
                   i, yrs, days, days_more;
       long  secs;
 static int  reg_days[12] = {0,31,59,90,120,151,181,212,243,273,304,334},
              lp_days[12] = {0,31,60,91,121,152,182,213,244,274,305,335};
       struct tm  *tp; 

  dp = date;   /* save */
  month[0] = *dp++;   month[1] = *dp++;   month[2] = '\0';  /* get "mm" */
  mm = atoi(month);                                   /* into mm as int */
  day[0] = *dp++;   day[1] = *dp++;   day[2] = '\0';        /* get "dd" */
  dd = atoi(day);                                     /* into dd as int */
  for(i=0; i<4; i++) year[i] = *dp++;
  year[4] = '\0';      /* get "yyyy" into */
  yyyy = atoi(year);   /*  yrs as an int  */
  yrs  =  yyyy - 1970;
  lp_yrs = yrs/4;   if(yrs%4 == 3) lp_yrs++;   reg_yrs  =  yrs - lp_yrs;
  days  =  365*reg_yrs + 366*lp_yrs;     /* = #(days) until given year */
  if(yyyy%4 == 0) /* yyyy = leap-year */
    days_more = lp_days[--mm];         /* = #(days) from start of yyyy */
  else  days_more = reg_days[--mm];                  /* until month mm */
  days  +=  days_more + dd;              /* = total of days altogether */
  secs  =  86400*days;         /* = #(secs) of whole days since 1/1/70 */
  tp =  localtime(&secs);
  printf("%s falls on %s.\n", date, names[tp->tm_wday]);
  return;
 }

 e.g.: weekday 091119189 reports that the date is a Monday.

 Hope this helps!

 Scott McCaughrin
 Dept. of Computer Science
 University of Illinois
 Urbana, Illinois. USA