If you'll recall, the tz package `mktime' normally returns -1 when given a time in a `spring-forward' gap, but the NIST-PCTS:151-2 test suite, Version 1.4, (1993-12-03), claims that this is a bug. In March I submitted a formal Defect Report about this to the committee responsible for the C Standard, and have just received a copy of the draft response (SC22/WG14 DR#136). This response has been approved by WG14 but is subject to a final review, so it is not official yet. The response reads as follows: The Standard does not specify the behavior precisely enough to preclude `mktime' from returning a value of `(time_t)-1' and leaving the `tm_isdst' member set to -1 in such situations. In other words, assuming this response becomes official as expected, PCTS must not insist that mktime yield a value other than -1 in such situations. Perhaps someone who's familiar with PCTS (which I am not) can forward this info to PCTS's maintainers. Here's the example program that I submitted as part of Defect Report #136. According to the draft response, a conforming implementation in a U.S. locale can print `mktime failed' when presented with the following program, since the requested time is in a `spring-forward' gap. #include <stdio.h> #include <time.h> int main() { struct tm t; time_t r; /* 1994-04-03 02:30:00 */ t.tm_year = 1994 - 1900; t.tm_mon = 3; t.tm_mday = 3; t.tm_hour = 2; t.tm_min = 30; t.tm_sec = 0; t.tm_isdst = -1; /* i.e. unknown */ r = mktime(&t); if (r == -1) printf("mktime failed\n"); else printf("%s", ctime(&r)); return 0; }