Notes

   If the routine call is made during a seasonal time zone change
   when the local time is indeterminate, an error is returned. For
   example, if the time zone change occurs at the current local time
   of 2:00 A.M. to a new local time of 1:00 A.M., and the routine is
   called between 1:00 A.M. and 2:00 A.M., it cannot be determined
   which TDF applies.