Use a reliable source for TZ offset

Both the POSIX and Linux man pages for localtime_r() state that
  localtime_r need not set the global variable timezone. I found
  that '\set_clock local' was using an offset of -0500 instead of
  -0400 (EDT). Change date_strget to use tm_gmtoff.

src/misc.c

@@ -2834,7 +2834,7 @@ char *date_strget(char *buf, int buflen, int localtime)
     if (localtime)
     {
         mytm = localtime_r(&t, &result);
-        mytimezone = timezone;
+        mytimezone = - (int)result.tm_gmtoff;
     }
     else
     {