[scottlamb]

There's another easy way to avoid this: use localtime_r instead of localtime. From the glibc source:

    /* Update internal database according to current TZ setting.
       POSIX.1 8.3.7.2 says that localtime_r is not required to set tzname.
       This is a good idea since this allows at least a bit more parallelism.  */
    tzset_internal (tp == &_tmbuf && use_localtime, 1);

mktime also does the tzset call every time, though:

    time_t
    mktime (struct tm *tp)
    {
    #ifdef _LIBC
      /* POSIX.1 8.1.1 requires that whenever mktime() is called, the
         time zone names contained in the external variable 'tzname' shall
         be set as if the tzset() function had been called.  */
      __tzset ();
    #endif

and I don't see any way around that other than setting TZ=: or some such.