Usenet 1994 October/usenetsourcesnewsgroupsinfomagicoctober1994disk2.iso/misc/volume6/isdst

Internet Message Format | 1989-02-03 | 3KB

Path: lll-winken!ames!mailrus!tut.cis.ohio-state.edu!ucbvax!unisoft!uunet!allbery
From: [e-mail] (Brandon S. Allbery - comp.sources.misc)
Newsgroups: comp.sources.misc
Subject: v06i037: Evaluate if a date is DST or not
Message-ID: <[e-mail]>
Date: 4 Feb 89 03:22:00 GMT
Sender: [e-mail]
Reply-To: [e-mail] (David Goodenough)
Lines: 77
Approved: [e-mail] (Brandon S. Allbery - comp.sources.misc)

Posting-number: Volume 6, Issue 37
Submitted-by: [e-mail] (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.
--
    [e-mail] - David Goodenough        +---+
                        IHS    | +-+-+
    ....... !harvard!xait!lakart!dg            +-+-+ |
AKA:    [e-mail]                +---+

--- 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