Conversion of year/month/day to day since epoch, and back (with weekday)

datol.cxx

#define	OFFS	-306		/* Offset to epoch-day (Day 1 is 1/1/0001) */
#define	WD	0L		/* Day of week offset */

/*
 * datol: takes a YMD date, and returns the number of days since some base
 * date (in the very distant past).
 *
 * Handy for getting date of x number of days before/after a given date.
 * A date is valid if ltodate() yields the same YMD you started with.
 *
 * Translated to C by Clifford Heath from Nat Howard's Fortran translation
 * of the Algol original by Robert G. Tantzen, as published in Collected
 * Algorithms of the ACM (CACM algorithm 199)
 */
static long
datol(
	int	year,
	int	mon,
	int	day
)
{
	if (mon < 3)				/* Adjust for February. */
		mon += 9, --year;
	else
		mon -= 3;

	return ((146097L * (year/100L)) / 4L)	/* Century days */
	     + ((1461L * (year%100)) / 4L)	/* Year days */
	     + ((153L*mon + 2L) / 5L)		/* Add adjustment for month */
	     + day				/* Add day */
	     + OFFS;				/* Add offset */
}

/*
 * ltodate: Convert day since epoch to date.
 *
 * Returns day of the week (0 = Sunday to 6 = Saturday)
 * and sets ymd[3]:
 * ymd[0] = year (actual year, like 1977, not 77 unless it was 77 a.d.);
 * ymd[1] = month;
 * ymd[2] = day of month;
 */
static int
ltodate(
	long	j,
	int	ymd[3]
)
{
	long	d, m, y, wd;

	wd = (j + WD)%7L;
	j -= OFFS;
	y = (4L * j - 1L)/146097L;
	j = 4L * j - 1L - 146097L * y;
	d = j/4L;
	j = (4L * d + 3L)/1461L;
	d = 4L * d + 3L - 1461L * j;
	d = (d + 4L)/4L;
	m = (5L * d - 3L)/153L;
	d = 5L * d - 3 - 153L * m;
	d = (d + 5L) / 5L;
	y = 100L * y + j;
	if (m < 10)
		m += 3;
	else {
		m -= 9;
		++y;
	}
	ymd[0] = (int) y;
	ymd[1] = (int) m;
	ymd[2] = (int) d;

	return wd;
}