Time zone conversion C API on Linux, anyone?

Question

I\'m looking for something that I presumed would be very simple - given local Unix time in a specific time zone (specified as a string, e.g., \"America/New_York\" - note tha

Answer 1

Wanted to add a bit more detail here.

If you try the following:

#include 
#include     /* defines 'extern long timezone' */

int main(int argc, char **argv)
{
    time_t t, lt, gt;
    struct tm tm;

    t = time(NULL);
    lt = mktime(localtime(&t));
    gt = mktime(gmtime(&t));

    printf( "(t = time(NULL)) == %x,\n"
        "mktime(localtime(&t)) == %x,\n"
        "mktime(gmtime(&t)) == %x\n"
        "difftime(...) == %f\n"
        "timezone == %d\n", t, lt, gt,
        difftime(gt, lt), timezone);
    return 0;
}

you'll notice that timezone conversions make sure that:

• mktime(localtime(t)) == t, and
• mktime(gmtime(t)) == t + timezone,
  therefore:
• difftime(mktime(gmtime(t)), mktime(localtime(t))) == timezone
  (the latter is a global variable initialized by either tzset() or the invocation of any timezone conversion function).

Example output of the above:

$ TZ=GMT ./xx
(t = time(NULL)) == 4dd13bac,
mktime(localtime(&t)) == 4dd13bac,
mktime(gmtime(&t)) == 4dd13bac
difftime(...) == 0.000000
timezone == 0

$ TZ=EST ./xx
(t = time(NULL)) == 4dd13baf,
mktime(localtime(&t)) == 4dd13baf,
mktime(gmtime(&t)) == 4dd181ff
difftime(...) == 18000.000000
timezone == 18000

$ TZ=CET ./xx
(t = time(NULL)) == 4dd13bb2,
mktime(localtime(&t)) == 4dd13bb2,
mktime(gmtime(&t)) == 4dd12da2
difftime(...) == -3600.000000
timezone == -3600

In that sense, you're attempting to "do it backwards" - time_t is treated as absolute in UN*X, i.e. always relative to the "EPOCH" (0:00 UTC on 01/01/1970).

The difference between UTC and the current timezone (last tzset() call) is always in the external long timezone global.

That doesn't get rid of the environment manipulation uglyness, but you can save yourself the effort of going through mktime().