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 
/**
 * This modifies the TZ environment variable and calls
 * tzset() to update the global timezone flags.  The
 * previous setting of TZ is returned.  An empty timezone
 * implies the system default value.
 *
 * Changing this affects all threads in this process.  See
 * zdump(1) to check timezone names.
 *
 * @param tzname Name of the new timezone ("US/Eastern").
 * @return The old value of TZ.
 */
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);