You can use this:
SET FEEDBACK OFF;
SET SERVEROUTPUT ON;
DECLARE
V_START_DATE CHAR(17) := '28/03/16 17:20:00';
V_END_DATE CHAR(17) := '30/03/16 17:50:10';
V_DATE_DIFF VARCHAR2(17);
BEGIN
SELECT
(TO_NUMBER( SUBSTR(NUMTODSINTERVAL(TO_DATE(V_END_DATE , 'DD/MM/YY HH24:MI:SS') - TO_DATE(V_START_DATE, 'DD/MM/YY HH24:MI:SS'), 'DAY'), 02, 9)) * 24) +
(TO_NUMBER( SUBSTR(NUMTODSINTERVAL(TO_DATE(V_END_DATE , 'DD/MM/YY HH24:MI:SS') - TO_DATE(V_START_DATE, 'DD/MM/YY HH24:MI:SS'), 'DAY'), 12, 2))) ||
SUBSTR(NUMTODSINTERVAL(TO_DATE(V_END_DATE , 'DD/MM/YY HH24:MI:SS') - TO_DATE(V_START_DATE, 'DD/MM/YY HH24:MI:SS'), 'DAY'), 14, 6) AS "HH24:MI:SS"
INTO V_DATE_DIFF
FROM
DUAL;
DBMS_OUTPUT.PUT_LINE(V_DATE_DIFF);
END;