I’ve gotten pretty careful about keeping time in UTC and then converting it to localtime for the user to understand. For the first time, I actually had to find the localtime in a non-local timezone. It’s ugly. It seems you have to mess with the TZ environment variable. Here’s what I wrote:

include 

include 

std::string changeTimeZone (const std::string &tzname)
{
    char *tzptr = getenv ("TZ");
    std::string  tzold;

if (tzptr)
    tzold = tzptr;
if (tzname.empty())
    unsetenv ("TZ");
else
    setenv ("TZ", tzname.c_str(), 1);
tzset();
return tzold;


}

And here’s a code fragment that uses it:

std::string oldzone = changeTimeZone("US/Pacific");
time_t  seconds = ::time(0);
struct tm   tm_time;


localtime_r (&seconds, tm_time);
changeTimeZone(oldzone);