Getting the difference between Dates

To get the difference between  two dates;
The below function return the interval time between two dates:

create or replace function 

get_elapsed (pstart in date , pend in date )

 return interval day to second
as
begin
    return (cast(pend as timestamp)- cast(pstart as timestamp))  DAY TO SECOND;
end ;

SELECT GET_ELAPSED(

to_date('19/11/2014 12:10:20', 'DD/MM/YYYY HH24:MI:SS'), to_date('19/11/2014 13:10:20', 'DD/MM/YYYY HH24:MI:SS') 

)  ELAPSED

 FROM DUAL;

ELAPSED:

+000000000 01:00:00

We can extract hour :

SELECT EXTRACT(hour from GET_ELAPSED(to_date('19/11/2014 12:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'),
                           to_date('19/11/2014 13:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'))) HOUR
  FROM DUAL;

HOUR:

1

We can extract hours,minutes and seconds 

SELECT
   GET_ELAPSED(to_date('19/11/2014 12:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'),
                           to_date('19/11/2014 13:00:00',
                                   'DD/MM/YYYY HH24:MI:SS') ) ELAPSED,
 
   EXTRACT(HOUR FROM GET_ELAPSED(to_date('19/11/2014 12:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'),
                           to_date('19/11/2014 13:00:00',
                                   'DD/MM/YYYY HH24:MI:SS')) ) HOUR,
             EXTRACT(MINUTE FROM GET_ELAPSED(to_date('19/11/2014 12:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'),
                           to_date('19/11/2014 13:00:00',
                                   'DD/MM/YYYY HH24:MI:SS')) ) MINUTE,      
           EXTRACT(SECOND FROM GET_ELAPSED(to_date('19/11/2014 12:10:20',
                                   'DD/MM/YYYY HH24:MI:SS'),
                           to_date('19/11/2014 13:00:00',
                                   'DD/MM/YYYY HH24:MI:SS')) ) SECOND                                          
  FROM DUAL;

    ELAPSED                 HOUR MINUTE SECOND

+000000000 00:49:40 0 49 40

You can refer to Database SQL Language Reference to get more details about Interval type  and extract function;