Path: utzoo!mnetor!uunet!steinmetz!davidsen
From: davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: day of week
Message-ID: <9404@steinmetz.steinmetz.UUCP>
Date: 3 Feb 88 15:16:02 GMT
References: <141900013@occrsh.ATT.COM>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 94

There has been so much commentary on the topic of "what day of the
week," that I am posting a short program to provide the information. It
returns a pseudo-julian date which may be used to uniquely identify any
date from Jan 1 1980 to Feb 29, 2100. One procedure takes the month,
day, year, and returns a unique number, the other does the inverse. The
number is such that num%7 = day_of_week, and 0 means Sunday.

Before I get a million postings, yes I have a real julian date routine.
This one works and returns values which will fit in a short as long as I
live.

/*
 *  JULI - pseudo julian date package
 *
 *  Convert any post-1980 date to an integer day number and back to
 *  day, month, year. This will work up to feb 29, 2100.
 */

static months[12][2] = {
      {0, 0}, {31, 31}, {59, 60}, {90, 91},
      {120, 121}, {151, 152}, {181, 182}, {212, 213},
      {243, 244}, {273, 274}, {304, 305}, {334, 335}};

int jdate (month, day, year)
    int month, day, year;
{
    register int iy, ly, lyf;

    iy = year - 1980;
    ly = (iy+3) / 4; /* leap year count */
    lyf = (iy%4 == 0);
    return (365*iy + ly + months[month-1][lyf] + day);
}

/*
 *  Convert day number to date
 */

unjuli (jd, month, day, year)
    int jd; /* julian day */
    int *month, *day, *year;
{
    register int
    lyf, /* leap year (this year) flag */
    dd, mm, /* calculated day and month */
    yy, ly; /* incremental years, leapyears */

  /* first guess as to year */
    yy = jd / 365;
    ly = (yy+3) / 4;
    while ((dd = jd - 365*yy - ly) <= 0) {
    /* correct for dates near the end of a year */
      yy--;
      ly = (yy+3) / 4;
    }

  /* find the month */
    lyf = (yy%4 == 0);
    mm = 0;
    while (months[mm][lyf] < dd && ++mm < 12);

  /* return values */
    *year = yy + 1980;
    *month = mm;
    *day = dd - months[mm-1][lyf];
}

/*
 *  Test routine for julian package
 */

#ifdef TEST
#include <stdio.h>

main () {
    int m, d, y, jd;

    for ( ; ; ) {
      printf ("Enter month, day, year: ");
      scanf ("%d%d%d", &m, &d, &y);
      if (y < 1900) y += 1900;
      jd = jdate (m, d, y);
      printf ("Juliam for %d/%d/%d = %d\n", m, d, y, jd);

      unjuli (jd, &m, &d, &y);
      printf ("Converted back = %d/%d/%d\n", m, d, y);
    }
}
#endif

-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me