How to get the current time zone?

Question

In most of the examples I had seen:

time_zone_ptr zone( new posix_time_zone("MST-07") ); 

But I just want to get the current time zone for the machine that runs the code. I do not want to hard code the time zone name.

Answer 1

Plain posix: call tzset, use tzname.

#include <ctime>
tzset();
time_zone_ptr zone(new posix_time_zone(tzname[localtime(0)->tm_isdst]));

Posix with glibc/bsd additions:

time_zone_ptr zone(new posix_time_zone(localtime(0)->tm_zone));

---

The above are abbreviated Posix timezones, defined in terms of offset from UTC and not stable over time (there's a longer form that can include DST transitions, but not political and historical transitions).

ICU is portable and has logic for retrieving the system timezone as an Olson timezone (snippet by sumwale):

// Link with LDLIBS=`pkg-config icu-i18n --libs`
#include <unicode/timezone.h>
#include <iostream>

using namespace U_ICU_NAMESPACE;

int main() {
  TimeZone* tz = TimeZone::createDefault();
  UnicodeString us;
  std::string s;

  tz->getID(us);
  us.toUTF8String(s);
  std::cout << "Current timezone ID: " << s << '\n';
  delete tz;
}

On Linux, ICU is implemented to be compatible with tzset and looks at TZ and /etc/localtime, which on recent Linux systems is specced to be a symlink containing the Olson identifier (here's the history). See uprv_tzname for implementation details.

Boost doesn't know how to use the Olson identifier. You could build a posix_time_zone using the non-DST and DST offsets, but at this point, it's best to keep using the ICU implementation. See this Boost FAQ.