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