SELECT * FROM test;

TIME1     TIME2
--------- ---------
27-OCT-14 17-OCT-14
01-OCT-14 01-OCT-14

Elapsed: 00:00:00.00

SELECT TO_CHAR(time1, 'DD-MON-YYYY HH:MI:SS') dt1, TO_CHAR(time2, 'DD-MON-YYYY HH:MI:SS') time2,
                 (time1 - time2) * 24 difference
  FROM test

DT1                     TIME2                    DIFFERENCE
--------------------   -------------------- ---------------
27-OCT-2014 12:00:00    17-OCT-2014 12:00:00             240
01-OCT-2014 12:33:00    01-OCT-2014 10:33:00               2


--------------------------2nd Example---------------------------------------------

Select floor((dt1-dt2)*24*60*60)/3600)||'HOURS'||floor((((dt1-dt2)*24*60*60) 
- 
floor(((dt1-dt2)*24*60*60)/3600)*3600)/60)||'MINUTES'||round((((dt1-dt2)*24*60*60) 
-
floor(((dt1-dt2)*24*60*60)/3600)*3600 -
(floor((((dt1-dt2)*24*60*60) -
floor(((dt1-dt2)*24*60*60)/3600)*3600)/60)*60)))||'SECS' time_difference from dual;

-------------------------3rd Example-------------------------------

DECLARE
   DH NUMBER;
   DM NUMBER;
   DS NUMBER;
   DMD1 CHAR(2);
   DMD2 CHAR(2);
BEGIN
    DH:=SUBSTR(:OUT_TIME,1,2)-SUBSTR(:IN_TIME,1,2);
    DM:=SUBSTR(:OUT_TIME,4,2)-SUBSTR(:IN_TIME,4,2);
    DS:=SUBSTR(:OUT_TIME,7,2)-SUBSTR(:IN_TIME,7,2);
    DMD1:=SUBSTR(:IN_TIME,10,2);
    DMD2:=SUBSTR(:OUT_TIME,10,2);
IF DMD1=DMD2 THEN
   :FINAL:=DH||':'||DM||':'||DS;
ELSE
   :FINAL:=DH+12||':'||DM||':'||DS;
END IF;
 END;

DECLARE
	RESULT NUMBER;
	STARTDATE DATE;
	FINISHDATE DATE;
	X VARCHAR2 (100);
	Y VARCHAR2 (100);
	SUBTRACTED_DATE DATE;
BEGIN
          X := to_char(:RGSFECHAFINPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAFINPDF,'HH24:MI');
	  Y := to_char(:RGSFECHAINIPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAINIPDF,'HH24:MI');
	  FINISHDATE := to_date(x,'DD/MM/YYYY HH24:MI');
	  STARTDATE := to_date(y,'DD/MM/YYYY HH24:MI');
	  RESULT := FINISHDATE - STARTDATE;
	  :RGSESTIMADOPDF := TO_CHAR(RESULT);
END;

select
   round(to_number(sysdate-(sysdate-1.041))*24)
from
   dual;

Result:

ROUND(TO_NUMBER(SYSDATE-(SYSDATE-1.041))*24)
--------------------------------------------
                                          25

DECLARE
	RESULT NUMBER;
	STARTDATE DATE;
	FINISHDATE DATE;
	X VARCHAR2 (16);
	Y VARCHAR2 (16);
	SUBTRACTED_DATE DATE;
BEGIN
		X := to_char(:RGSFECHAFINPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAFINPDF,'HH24:MI');
	  Y := to_char(:RGSFECHAINIPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAINIPDF,'HH24:MI');
	  FINISHDATE := to_date(x,'DD/MM/YYYY HH24:MI');
	  STARTDATE := to_date(y,'DD/MM/YYYY HH24:MI');
	  /*--RESULT := FINISHDATE - STARTDATE;
	  --:RGSESTIMADOPDF := TO_CHAR(RESULT);*/
	  SELECT FLOOR ((FINISHDATE-STARTDATE)*24)|| ':' ||(MOD((FINISHDATE-STARTDATE) * 1440,60)|| ':' ||
		MOD((FINISHDATE-STARTDATE)*86400,60)) INTO :RGSESTIMADOPDF FROM RGSORDMNT;
END;

DECLARE
	RESULT NUMBER;
	STARTDATE DATE;
	FINISHDATE DATE;
	X VARCHAR2(100);
	Y VARCHAR2(100);
	M1 VARCHAR2(100);
BEGIN
	X := to_char(:RGSFECHAFINPDF,'DD/MM/YYYY')||' '||to_char(:RGSHRAFINPDF,'HH24:MI');
	Y := to_char(:RGSFECHAINIPDF,'DD/MM/YYYY')||' '||to_char(:RGSHRAINIPDF,'HH24:MI');
  FINISHDATE := to_date(X,'DD/MM/YYYY HH24:MI');
	STARTDATE := to_date(Y,'DD/MM/YYYY HH24:MI');
	SELECT floor((FINISHDATE - STARTDATE)*24)||':'||
				 ROUND((MOD((FINISHDATE - STARTDATE)*1440,60)),2)||':'||
				 ROUND((MOD((FINISHDATE - STARTDATE)*86400,60)),2)
	INTO :RGSESTIMADOPDF FROM RGSORDMNT;
/*EXCEPTION
	WHEN OTHERS THEN
	MESSAGE('SOMETHING IS MISSING... TRY AGAIN');*/
END;

BEGIN
   :YOUR_BLOCK.ESTIMATED := ROUND(24 * 
   to_date(to_char(:YOUR_BLOCK.DATE_STARTED||' '||:YOUR_BLOCK.HOUR_STATED,'MM-DD-YYYY HH24:mi'),'MM-DD-YYYY HH24:mi') 
   - to_date(to_char(:YOUR_BLOCK.DATE_ENDED||' '||:YOUR_BLOCK.HOUR_ENDED,'MM-DD-YYYY HH24:mi'),'MM-DD-YYYY HH24:mi') 
   );
END;

BEGIN
DECLARE
	RESULT NUMBER;
	STARTDATE DATE;
	FINISHDATE DATE;
	STARTDATE2 DATE;
	FINISHDATE2 DATE;
	STARTDATE3 DATE;
	FINISHDATE3 DATE;
	X VARCHAR2 (100);
	Y VARCHAR2 (100);
	SUBTRACTED_DATE DATE;
BEGIN
		X := to_char(:RGSFECHAFINPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAFINPDF,'HH24:MI');
	  Y := to_char(:RGSFECHAINIPDF,'DD/MM/YYYY')||' '|| to_char(:RGSHRAINIPDF,'HH24:MI');
	  FINISHDATE := to_date(x,'DD/MM/YYYY HH24:MI');
	  STARTDATE := to_date(y,'DD/MM/YYYY HH24:MI');
	  SELECT FLOOR ((FINISHDATE-STARTDATE)*24)|| ':' ||round((MOD((FINISHDATE-STARTDATE) * 1440,60)),0)|| ':' ||
		round((MOD((FINISHDATE-STARTDATE)*86400,60)),0) INTO :RGSESTIMADOPDF FROM RGSORDMNT
		where 
		RGSCODCIA = SUBSTR(:CIASUC,1,3) and
		RGSCODSUC = SUBSTR(:CIASUC,4,3) and
		RGSNUMSOL = :RGSNUMSOL;
		Commit;
EXCEPTION
	WHEN OTHERS THEN
	MESSAGE ('RECUERDA, PRIMERO SE GRABA Y DESPUES DE CALCULA O DEBE FALTARTE ALGUN CAMPO FECHA U HORA DE PADIF');
END;

:RGSESTIMADOPDF := FLOOR ((FINISHDATE-STARTDATE)*24)|| ':' ||floor((MOD((FINISHDATE-STARTDATE) * 1440,60)),0)|| ':' ||
                   floor((MOD((FINISHDATE-STARTDATE)*86400,60)),0);