Timezones

Different countries (and sometimes even different regions within a single country) operate on different timezones and DST regimes. Programs that input and output times must take into account the timezone and DST regime of the system on which they are run. Fortunately, all of the details are handled by the C library.

char *tzname[2];    /* Name of timezone and alternate (DST) timezone */
int daylight;       /* Nonzero if there is an alternate (DST) timezone */
long timezone;      /* Seconds difference between UTC and local
                       standard time */

The tzset() function first checks the TZ environment variable. If this variable is not set, then the timezone is initialized to the default defined in the timezone file /etc/localtime. If the TZ environment variable is defined with a value that can’t be matched to a timezone file, or it is an empty string, then UTC is used. The TZDIR environment variable (a nonstandard GNU-extension) can be set to the name of a directory in which timezone information should be sought instead of in the default /usr/share/zoneinfo.

We can see the effect of the TZ variable by running the program in Example 10-4. In the first run, we see the output corresponding to the system’s default timezone (Central European Time, CET). In the second run, we specify the timezone for New Zealand, which at this time of year is on daylight saving time, 12 hours ahead of CET.

$ ./show_time
ctime() of time() value is:  Tue Feb  1 10:25:56 2011
asctime() of local time is:  Tue Feb  1 10:25:56 2011
strftime() of local time is: Tuesday, 01 Feb 2011, 10:25:56 CET
$ TZ=":Pacific/Auckland" ./show_time
ctime() of time() value is:  Tue Feb  1 22:26:19 2011
asctime() of local time is:  Tue Feb  1 22:26:19 2011
strftime() of local time is: Tuesday, 01 February 2011, 22:26:19 NZDT

Example 10-4. Demonstrate the effect of timezones and locales

time/show_time.c
#include <time.h>
#include <locale.h>
#include "tlpi_hdr.h"

#define BUF_SIZE 200

int
main(int argc, char *argv[])
{
    time_t t;
    struct tm *loc;
    char buf[BUF_SIZE];

    if (setlocale(LC_ALL, "") == NULL)
        errExit("setlocale");   /* Use locale settings in conversions */

    t = time(NULL);

    printf("ctime() of time() value is:  %s", ctime(&t));

    loc = localtime(&t);
    if (loc == NULL)
        errExit("localtime");

    printf("asctime() of local time is:  %s", asctime(loc));

    if (strftime(buf, BUF_SIZE, "%A, %d %B %Y, %H:%M:%S %Z", loc) == 0)
        fatal("strftime returned 0");
    printf("strftime() of local time is: %s\n", buf);

    exit(EXIT_SUCCESS);
}
      time/show_time.c

SUSv3 defines two general ways in which the TZ environment variable can be set. As just described, TZ can be set to a character sequence consisting of a colon plus a string that identifies the timezone in an implementation-specific manner, typically as a pathname for a timezone description file. (Linux and some other UNIX implementations permit the colon to be omitted when using this form, but SUSv3 doesn’t specify this; for portability, we should always include the colon.)

The other method of setting TZ is fully specified in SUSv3. In this method, we assign a string of the following form to TZ:

std offset [ dst [ offset ][ , start-date [
 /time ] , end-date [ /time ]]]

Spaces are included in the line above for clarity, but none should appear in the TZ value. The brackets ([]) are used to indicate optional components.

The std and dst components are strings that define names for the standard and DST timezones; for example, CET and CEST for Central European Time and Central European Summer Time. The offset in each case specifies the positive or negative adjustment to add to the local time to convert it to UTC. The final four components provide a rule describing when the change from standard time to DST occurs.

The dates can be specified in a variety of forms, one of which is Mm.n.d. This notation means day d (0 = Sunday, 6 = Saturday) of week n (1 to 5, where 5 always means the last d day) of month m (1 to 12). If the time is omitted, it defaults to 02:00:00 (2 AM) in each case.

Here is how we could define TZ for Central Europe, where standard time is one hour ahead of UTC, and DST—running from the last Sunday in March to the last Sunday in October—is 2 hours ahead of UTC:

TZ="CET-1:00:00CEST-2:00:00,M3.5.0,M10.5.0"

We omitted the specification of the time for the DST changeover, since it occurs at the default of 02:00:00. Of course, the preceding form is less readable than the Linux-specific near equivalent:

TZ=":Europe/Berlin"

Timezones

Timezone definitions

Note

Specifying the timezone for a program