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From: dml@loral.UUCP (Dave Lewis)
Newsgroups: net.lang.c
Subject: Re: Calendar Functions
Message-ID: <1229@loral.UUCP>
Date: Wed, 10-Sep-86 12:47:34 EDT
Article-I.D.: loral.1229
Posted: Wed Sep 10 12:47:34 1986
Date-Received: Thu, 11-Sep-86 22:43:22 EDT
References: <206@cascade.STANFORD.EDU>
Reply-To: dml@loral.UUCP (Dave Lewis)
Distribution: net.lang.c
Organization: Loral Instrumentation, San Diego
Lines: 97
Keywords: Calendars
Summary: Here are some bones, string 'em together

In article <206@cascade.STANFORD.EDU> leeke@cascade.UUCP (Steven D. Leeke)
writes:
>Could someone please give me some pointers to functions for creating
>calendars? e.g. for any day from 1900 to as far in the future as possible.
>Actual code would be GREATLY appreciated - C preferred.
>

  Here's a collection of calendar functions; writing a main() driver
function should be trivial. These were originally assignments for a Pascal
class (what the hey, an easy 4 credits with an A+) and I rewrote them in
C for the hell of it. Good for contrasting Pascal and C - things I had to
work around in Pascal came out as straight code in C.

/*  Determines whether a given year is a leap year
 */

leapyear (year)
int year;
{
  if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0))
     return (1);
  else
     return (0);
}

/*  Returns #days in month given month and year, taking
 *    leap years into account for February.
 */

daysinmonth (month, year)
int month, year;
{
  if (month == 2)          /* Is it February?             */
     if (leapyear (year))  /* If so, is it a leap year?   */
	return (29);       /* 29 days in Feb in leap year */
     else
	return (28);       /* 28 days if not              */
  else{
     if (month > 7)        /* Is it August -> December?   */
	month++;           /* Invert even/odd state if so */
     if (month & 1)        /* Odd months have 31 days     */
	return (31);
     else
	return (30);       /* Even months have 30 days    */
  }
}

/*  Determines whether a given date is valid
 */

validdate (month, day, year)
int month, day, year;
{
  if (month < 1 || month > 12 || day < 1 ||
     day > daysinmonth (month, day, year) ||
     year < 1583 || year > 9999)
	return (0);
  else
	return (1);
}

/*  Given a valid date (month, day, and year) Zeller will
 *    return an integer representing the day of week that
 *    date will fall on. 0 = Sunday, 6 = Saturday.
 */

zeller (month, day, year)
int month, day, year;
{
  int century;
  month -= 2;       /* Years start on March 1 so adjust standard date */
  if (month < 1) {
     month += 12;
     year--;
  }
  century = year / 100;
  year = year % 100;
  return (((2.6 * month - 0.1) + day + year + year / 4
	   + century / 4 - century * 2) % 7);
}

  These functions will work for any date between March 1, 1583 and December 31,
9999. Have fun!

-------------------------------
		Dave Lewis    Loral Instrumentation   San Diego

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-------------------------------