Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!tektronix!psueea!psueea.uucp!kirkenda
From: kirkenda@psueea.uucp (Steve Kirkendall)
Newsgroups: comp.sys.ibm.pc
Subject: Re: calendar code
Message-ID: <1428@psueea.UUCP>
Date: 30 Jun 89 17:18:31 GMT
References: <82362@ti-csl.csc.ti.com>
Sender: news@psueea.UUCP
Reply-To: kirkenda@jove.cs.pdx.edu (Steve Kirkendall)
Organization: Dept. of Computer Science, Portland State University; Portland OR
Lines: 243


	Sorry for posting this, But I had trouble sending email.

	Although I suppose this is probably of general interest anyway...

In article <82362@ti-csl.csc.ti.com> Donahue@TILDE (Them bats is smart; they use radar!) writes:
>
>I'm looking for a Turbo C function that, when given
>an arbitrary date, will return the day of the week that the date
>falls on.
>
>E.G.  given 11 6 1991 [nov 6, 1991], the function
>      returns WEDNESDAY (or 3, if SUNDAY = 0).

This is slight overkill, but the rest of this message is a shar archive which
contains an implementation of the UNIX SysV localtime() function, and another
function called cvtdate() which converts and ASCII date string to UNIX format.

Cvtdate() is called with two arguments: the date in "YYMMDD" format, and the
time in "hhmm" format.  It returns a long int value which is the number of
seconds since time began (i.e. Jan 1, 1970).

Localtime() is called with a single argument: a pointer to a long int variable
which contains a time value.  It returns a pointer to a structure which breaks
the time down into year, month, day of year, day of month, day of week, hours
minutes, and seconds.  This structure type is called "struct tm" and is declared
in time.h (included).

An example:
	#include "time.h"
	extern long	cvtdate();

	main()
	{
		long		when;
		struct tm	*tptr;


		when = cvtdate("911106", "0000");
		tptr = localtime(&when);
		printf("Day of week = %d\n", tptr->tm_wday);
	}

If all you really care about is day-of-week, you could modify cvtdate to count
days instead of seconds, and take (days % 7) to get the day of the week.


#	This is a shell archive.
#	Remove everything above and including the cut line.
#	Then run the rest of the file through sh.
-----cut here-----cut here-----cut here-----cut here-----
#!/bin/sh
# shar:	Shell Archiver
#	Run the following text with /bin/sh to create:
#	cvtdate.c
#	localtime.c
#	time.h
# This archive created: Thu Jun 29 14:54:18 1989
cat << \SHAR_EOF > cvtdate.c
/* cvtdate.c */

/* This file contains the cvtdate() function, which converts a date string
 * into a UNIX-style time value.
 */

/* Offset into the year for 1st of each month (disregarding leap years) */
static int monthoff[12]={ 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
static int leapoff[12] ={ 0, 31, 60, 91, 121, 152, 182, 212, 244, 274, 305, 335 };

#define MIN	60L
#define HOUR	(60L*MIN)
#define DAY	(24L*HOUR)
#define YEAR	(365L*DAY) /* adjust for leapyears later */

static int cvt2(digits)
	char	*digits;
{
	return digits[0] * 10 + digits[1] - ('0' * 11);
}

long cvtdate(date, time)
	char	*date;
	char	*time;
{
	long	now;
	int	t;

	/* seconds for each year since 1970 */
	t = cvt2(date);
	if (t < 70)
		t += 100;
	now = (t - 70) * YEAR;

	/* adjust for intervening leapyears */
	now += ((t - 69) >> 2) * DAY;
#ifdef TEST
	printf("%ld + %ld = %ld\n", (t-70)*YEAR, now-(t-70)*YEAR, now);
#endif

	/* add in the month - adjust if this year is leapyear */
	now += ((t&3) ? monthoff : leapoff)[cvt2(date + 2) - 1] * DAY;

	/* add in the day, hour, and minute */
	now += (cvt2(date + 4) - 1) * DAY
	     + cvt2(time) * HOUR
	     + cvt2(time + 2) * MIN;

	return now;
}


#ifdef TEST
extern char *ctime();
main(argc, argv)
	int	argc;
	char	**argv;
{
	long	now;

	if (argc != 3)
	{
		printf("usage: %s yymmdd hhmm\n", argv[0]);
		exit(2);
	}
	
	now = cvtdate(argv[1], argv[2]);
	printf("now = %ld = \"%s\";\n", now, ctime(&now));
}
#endif
SHAR_EOF
cat << \SHAR_EOF > localtime.c
/* localtime.c */

/* This file contains the source to a SysV-compatiple localtime() function
 * Written by Steve Kirkendall, placed in public domain     April,1989
 */


#include <time.h>

/* Offset into the year for 1st of each month (disregarding leap years) */
int monthoff[12]={ 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
int leapoff[12] ={ 0, 31, 60, 91, 121, 152, 182, 212, 244, 274, 305, 335 };

#define MIN	60L
#define HOUR	(60L*MIN)
#define DAY	(24L*HOUR)
#define YEAR	(365L*DAY) /* adjust for leapyears later */

struct tm *localtime(clock)
	long *clock;		/* the clock value to convert */
{
	static struct tm t;	/* the returned structure is STATIC!!! */
	long	timeleft;	/* amount of time not consumed in calc so far */
	int	i;

	/* find the current year, allowing for preceding leapyears */
	i = *clock / YEAR;	/* year - accurate formula 'til ~doomsday */
	t.tm_year = i + 1970;	/* add '1970' year offset */
	timeleft= *clock-i*YEAR;/* beginning of year, ignoring leapyears */
	i = (i + 1) / 4;	/* leap days, not including this year's */
	timeleft -= i * DAY;	/* use up all those leap days */

	/* calculate day of the year */
	t.tm_yday = timeleft / DAY;

	/* calculate month and day of month.  This might be a leap year */
	if (t.tm_year % 4 == 0)
	{
		/* use leapoff[] to find month */
		for (i = 11; leapoff[i] > t.tm_yday; i--)
		{
		}
		t.tm_mday = t.tm_yday - leapoff[i] + 1;
	}
	else
	{
		/* use regular monthoff[] table */
		for (i = 11; monthoff[i] > t.tm_yday; i--)
		{
		}
		t.tm_mday = t.tm_yday - monthoff[i] + 1;
	}
	t.tm_mon = i;
	timeleft -= t.tm_yday * DAY;

	/* calculate weekday. */
	t.tm_wday = (*clock / DAY + 4) % 7;

	/* calculate hour, minute, second */
	t.tm_hour = timeleft / HOUR;
	timeleft -= HOUR * t.tm_hour;
	t.tm_min = timeleft / MIN;
	timeleft -= MIN * t.tm_min;
	t.tm_sec = timeleft;

	/* calculate DST flag */
	t.tm_isdst = 0;

	return &t;
}

#ifdef TEST
main(argc, argv)
	int	argc;
	char	**argv;
{
	long	shot;	/* will this time be converted right? */
	struct tm *t;	/* the converted time */

	time(&shot);
	if (argc > 1)
	{
		shot += DAY * atoi(argv[1]);
	}
	t = localtime(&shot);
	printf("ctime() = \"%s\"\n", ctime(&shot));
	printf("tm_sec=%d\ntm_min=%d\ntm_hour=%d\ntm_mday=%d\ntm_mon=%d\n",
	     t->tm_sec, t->tm_min, t->tm_hour, t->tm_mday, t->tm_mon);
	printf("tm_year=%d\ntm_wday=%d\ntm_yday=%d\ntm_isdst=%d\n",
	     t->tm_year, t->tm_wday, t->tm_yday, t->tm_isdst);
}
#endif
SHAR_EOF
cat << \SHAR_EOF > time.h
struct tm {
	int	tm_sec;
	int	tm_min;
	int	tm_hour;
	int	tm_mday;
	int	tm_mon;
	int	tm_year;
	int	tm_wday;
	int	tm_yday;
	int	tm_isdst;
};

extern struct tm *localtime();
SHAR_EOF
#	End of shell archive
exit 0
	-- Steve Kirkendall
	   ...uunet!tektronix!psu-cs!kirkenda