Linux-x64 glibc: Why does Feb 1 come before Jan 31?

Question

When you call mktime(), Feb 1 seems to come before Jan 31. Why is this? Am I doing something wrong or is this a bug in glibc?

Here's the code:

struct tm tm;
time_t tt;

memset(&tm, 0, sizeof(tm));
tm.tm_year = 2011;
tm.tm_mon = 1;
tm.tm_mday = 31;
tm.tm_hour = 11;
tm.tm_min = 41;
tm.tm_sec = 28;
tm.tm_isdst = 0;
tt = mktime(&tm);

printf("Time now %d-%d-%d %d:%d:%d (%s) = %lu\n",
    tm.tm_year, tm.tm_mon, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec, tm.tm_zone, tt);


memset(&tm, 0, sizeof(tm));
tm.tm_year = 2011;
tm.tm_mon = 2;
tm.tm_mday = 1;
tm.tm_hour = 1;
tm.tm_min = 1;
tm.tm_sec = 1;
tm.tm_isdst = 0;
tt = mktime(&tm);

printf("Time now %d-%d-%d %d:%d:%d (%s) = %lu\n",
    tm.tm_year, tm.tm_mon, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec, tm.tm_zone, tt);

And here's the output:

Time now 2011-2-3 11:41:28 (PST) = 61257325288
Time now 2011-2-1 1:1:1 (PST) = 61257114061

Note that the original intention was to compare two time_t's. This issue causes the first date/time to appear to be later than the second, which is obviously a bit of a problem.

This is just compiled with "gcc test.c" and run with "./a.out" on Ubuntu 9.10, gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu8), libc-2.10.1-0ubuntu15

On a 32-bit system the results are as expected - i.e. completely different to the 64 bit result!

Would anyone care to confirm/refute this result and/or give some insight into what I may be doing wrong?

Answer 1

tm_mon is zero-based, so you attempted to set February 31st, which got normalized. Here's a link to the definition of mktime().