allbery@uunet.UU.NET (Brandon S. Allbery - comp.sources.misc) (02/04/89)
Posting-number: Volume 6, Issue 37
Submitted-by: dg@lakart.UUCP (David Goodenough)
Archive-name: isdst
[Does this critter deal with DST ambiguities like the recent change? ++bsa]
For a project I'm doing, I needed to know if a given date was Daylight
Savings or not. After searching long and hard, I dicovered that in the
United States at least, the changes are always the last Sunday of April
and October. The following algorithm will take a date in day, month,
year, century form, and tell if it is DST or not: it returns zero if
not DST, and 1 if the date is DST. Caveat: it assumes that the
changeover is at midnight: it doesn't understand about the time slip at
2AM. However as long as you aren't up at 2AM I don't predict this will
be a problem. As a freebie, it also includes a function that will
evaluate the day of the week for a given date. This function returns a
number between 0 and 6, where 0 means Sunday, 1 is Monday etc., and 6
is Saturday.
--
dg@lakart.UUCP - David Goodenough +---+
IHS | +-+-+
....... !harvard!xait!lakart!dg +-+-+ |
AKA: dg%lakart.uucp@xait.xerox.com +---+
--- cut here --- cut here --- cut here --- cut here --- cut here ---
#! /bin/sh
# This file was wrapped with "dummyshar". "sh" this file to extract.
# Contents: isdst.c
echo extracting 'isdst.c'
if test -f 'isdst.c' -a -z "$1"; then echo Not overwriting 'isdst.c'; else
sed 's/^X//' << \EOF > 'isdst.c'
Xint isdst(day, month, year, cent)
Xint day, month, year, cent;
X {
X if (month < 4 || month > 10)
X return(0); /* months 1 - 3, and 11, 12 are always not DST */
X if (month > 4 && month < 10)
X return(1); /* months 5 - 9 are always DST */
X
X /* see if there's a Sunday in the month after our day. */
X
X while (++day <= ((month == 4) ? 30 : 31))
X if (dayofweek(day, month, year, cent) == 0)
X return(month == 10); /* found one: that means we're before the
X * changeover, so 4 => not DST, 10 => DST */
X
X return(month == 4); /* failed, so we're after the changeover,
X * so 4 => DST, 10 => not DST */
X }
X
Xint dayofweek(day, month, year, cent)
Xint day, month, year, cent;
X {
X /* This has it's roots in the Roman calendar, where the first
X * month was Mar, and Jan and Feb were months 11 and 12 of
X * the previous year */
X if ((month = month - 2) < 1) /* back month by two, but check for wrap */
X {
X month = month + 12; /* reset to end of last year */
X if (year-- == 0) /* reduce year number */
X {
X year = 99; /* and adjust for century wrap if needed */
X cent--;
X }
X }
X return(((13 * month - 1) / 5 + day + year + year / 4 +
X cent / 4 - 2 * cent) % 7);
X /* I don't have a clue how this got dreamed
X * up, but it works. If memory serves I first
X * saw it in the British Science Museum, in
X * London, England */
X }
EOF
chars=`wc -c < 'isdst.c'`
if test $chars != 1425; then echo 'isdst.c' is $chars characters, should be 1425 characters!; fi
fi
exit 0