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.