SELECT fr_tm, to_tm
     , TO_CHAR(TO_DATE(sec, 'sssss'), 'hh24mi') tm
  FROM (SELECT fr_tm, to_tm
             , ROUND(
               (TO_DATE(to_tm, 'hh24mi') - TO_DATE(fr_tm, 'hh24mi'))
               *24*60*60
               ) sec
          FROM (SELECT '0900' fr_tm, '1530' to_tm FROM dual)
        )
;

감사합니다 꾸벅