Converting string containing localtime into UTC in C

Question

I have a string containing a local date/time and I need to convert it to a time_t value (in UTC) - I\'ve been trying this:

char* date = \"2009/09/01/00\";
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Answer 1

You can use mktime to interpret a struct tm in the local timezone. When you do so, be careful to set the tm_isdst flag. It's 0 for summertime, 1 for wintertime, and to -1 to have mktime figure it out. Here's some example code:

void main()
{
    char* date = "2009/09/01/00";
    struct tm cal = {};
    // Read string into struct tm
    strptime(date, "%Y/%m/%d/%H", &cal);
    // Tell mktime to figure out the daylight saving time
    cal.tm_isdst = -1;
    printf("%20s: %s", "Before mktime", asctime(&cal));
    // Convert struct tm to time_t
    time_t t = mktime(&cal);
    // Convert time_t to localtime
    struct tm localcal = *localtime(&t);
    printf("%20s: %s", "Local time", asctime(&localcal));
    printf("%20s: %i\n", "Local DST", localcal.tm_isdst);
    // Convert time_t to GMT
    struct tm gmcal = *gmtime(&t);
    printf("%20s: %s", "GM time", asctime(&gmcal));
    printf("%20s: %i\n", "GM DST", gmcal.tm_isdst);
}

This prints (I live in GMT+1, and it's wintertime now):

   Before mktime: Tue Sep  1 00:00:00 2009
      Local time: Tue Sep  1 00:00:00 2009
       Local DST: 1
         GM time: Mon Aug 31 22:00:00 2009
          GM DST: 0

It looks like mktime converts a date in September based on the current daylight savings time. It's November now, so it's actually one hour off. I haven't found a way to correct that.