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From: Makey@LOGICON.arpa (Jeff Makey)
Newsgroups: comp.unix.wizards
Subject: Daylight Savings Time: how to fix ctime.c
Message-ID: <4992@brl-adm.ARPA>
Date: Tue, 17-Mar-87 03:01:43 EST
Article-I.D.: brl-adm.4992
Posted: Tue Mar 17 03:01:43 1987
Date-Received: Wed, 18-Mar-87 05:49:55 EST
Sender: news@brl-adm.ARPA
Lines: 58

Those of you with UNIX source who plan to fix your ctime routine so it
knows about the 1987+ daylight savings time rules should be interested
in the following information about daylight time in the United States.

    before 1967     no daylight time at all
    1967 -> 1973    last Sunday in April -> last Sunday in October
    1974            first Sunday in January -> last Sunday in October
    1975            last Sunday in February -> last Sunday in October
    1976 -> 1986    last Sunday in April -> last Sunday in October
    beginning 1987  first Sunday in April -> last Sunday in October

Note that every UNIX system I have ever seen (including SVR2 and
4.2BSD) mistakenly thinks that daylight time in 1974 ends on the last
Sunday in November.  My evidence to the contrary is on page 741 of the
1984 edition of The World Almanac and Book of Facts and in a small
article on page 3 of the 27 October 1974 edition of the Los Angeles
Times (as recorded on microfilm at the San Diego city library).  Check
it yourself if you don't beleive me.

The following data structure is useful for determining when daylight
savings time is in effect in the United States.  Any future changes
should be simple and efficient.

struct dstab {
	int	dayyr;
	int	daylb;
	int	dayle;
} usdaytab[] {
	{ 1987,	96,	303 },	/* first Sun in Apr to last Sun in Oct */
	{ 1976,	119,	303 },	/* last Sun in Apr to last Sun in Oct */
	{ 1975,	58,	303 },	/* last Sun in Feb to last Sun in Oct */
	{ 1974,	5,	303 },	/* first Sun in Jan to last Sun in Oct */
	{ 1967,	119,	303 },	/* last Sun in Apr to last Sun in Oct */
	{ 0,	400,	0 }	/* no daylight time at all */
};

The order (years are decreasing) of the entries in the table is
important.  In your code, use something like the following to select
the appropriate table entry:

	struct dstab *ds;
	for (ds = usdaytab; ds->dayyr; ds++)
		if (year >= ds->dayyr)
			break;

Those of you with 4.2BSD will notice a for loop very much like this one
in localtime().  The important difference is the ">=" rather than the
"==" in the if statement.

You have a little less than 3 weeks to change ctime and relink programs
that use it.  I used what(I) on 4.2BSD to find about 70 executables
that are affected.

                         :: Jeff Makey
                            Makey@LOGICON.ARPA

P.S.  Any requests for complete ctime sources will be forwarded to
      /dev/null.