The strange Chicago behavior occurs because for a few months back in 1936 Chicago switched to using Eastern Standard Time; mktime ended up using this information when trying to do a DST correction. The change below should "fix" things; it convinces mktime to work temporally backward through time adjustments that have been used in an area when trying to do a DST correction. (Of course if you really did start out with a 1936 time this is the wrong thing to do, but that's fairly unlikely.) --ado ------- localtime.c ------- *** /tmp/geta23340 Fri Nov 28 12:10:57 2003 --- /tmp/getb23340 Fri Nov 28 12:10:57 2003 *************** *** 5,11 **** #ifndef lint #ifndef NOID ! static char elsieid[] = "@(#)localtime.c 7.76"; #endif /* !defined NOID */ #endif /* !defined lint */ --- 5,11 ---- #ifndef lint #ifndef NOID ! static char elsieid[] = "@(#)localtime.c 7.77"; #endif /* !defined NOID */ #endif /* !defined lint */ *************** *** 1503,1508 **** --- 1503,1513 ---- register time_t t; register const struct state * sp; register int samei, otheri; + register int sameind, otherind; + register int i; + register int nseen; + int seen[TZ_MAX_TYPES]; + int types[TZ_MAX_TYPES]; int okay; if (tmp->tm_isdst > 1) *************** *** 1536,1545 **** if (sp == NULL) return WRONG; #endif /* defined ALL_STATE */ ! for (samei = sp->typecnt - 1; samei >= 0; --samei) { if (sp->ttis[samei].tt_isdst != tmp->tm_isdst) continue; ! for (otheri = sp->typecnt - 1; otheri >= 0; --otheri) { if (sp->ttis[otheri].tt_isdst == tmp->tm_isdst) continue; tmp->tm_sec += sp->ttis[otheri].tt_gmtoff - --- 1541,1560 ---- if (sp == NULL) return WRONG; #endif /* defined ALL_STATE */ ! for (i = 0; i < sp->typecnt; ++i) ! seen[i] = FALSE; ! nseen = 0; ! for (i = sp->timecnt - 1; i >= 0; --i) ! if (!seen[sp->types[i]]) { ! seen[sp->types[i]] = TRUE; ! types[nseen++] = sp->types[i]; ! } ! for (sameind = 0; sameind < nseen; ++sameind) { ! samei = types[sameind]; if (sp->ttis[samei].tt_isdst != tmp->tm_isdst) continue; ! for (otherind = 0; otherind < nseen; ++otherind) { ! otheri = types[otherind]; if (sp->ttis[otheri].tt_isdst == tmp->tm_isdst) continue; tmp->tm_sec += sp->ttis[otheri].tt_gmtoff - -----Original Message----- From: tz-request@lecserver.nci.nih.gov [mailto:tz-request@lecserver.nci.nih.gov] On Behalf Of Olson, Arthur David (NIH/NCI) Sent: Tuesday, November 25, 2003 7:40 AM To: Tz (tz@elsie.nci.nih.gov) Subject: FW: America/Chicago zone vs mktime w/tm_isdst!=-1 Dan Nelson is not on the time zone mailing list; direct replies appropriately. --ado -----Original Message----- From: Dan Nelson [mailto:dnelson@allantgroup.com] Sent: Monday, November 24, 2003 6:23 PM To: tz@lecserver.nci.nih.gov Subject: America/Chicago zone vs mktime w/tm_isdst!=-1 Here's an interesting problem. Usually when mktime() is called with tm_isdst set to 0 or 1, it will interpret the original time as being either in standard or daylight time according to tm_isdst, and update the resulting time_t value (and the struct tm) if the DST flag was wrong. This doesn't seem to happen with the America/Chicago zone. I've attached a simple program that passes three struct tm's to mktime, with three values of tm_isdst each. For most zones, the first and last times change depending on what value of tm_isdst is passed in. For America/Chicago, it looks like tm_isdst is ignored. Is it used correctly for the middle time (since it's in the fall-back range). Someone filed a FreeBSD PR against this back in June ( http://www.freebsd.org/cgi/query-pr.cgi?pr=53899 ), and I've verified the behaviour back to FreeBSD 4.0 (which had imported tzcode1999a). The classictzcode.tar.gz on elsie.nci.nih.gov looks at tm_isdst in all cases, so the change happened sometime between 1994 and 1999. I don't know if there are archives of tzcode sources, or I could pinpoint the exact release. It's not strictly a bug since POSIX basically says that any tm_isdst != -1 results in implementation-defined behaviour, but it's troublesome if the behaviour changes from zone to zone, and the current releases of Solaris, Tru64, AIX, and glibc(Linux) are consistent across timezones (they all adjust the incoming struct tm based on tm_isdst). And, a couple of POSIX/C99 links for the archives: http://www.opengroup.org/onlinepubs/007904975/functions/mktime.html http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/dr_280.htm -- Dan Nelson dnelson@allantgroup.com