CREATE OR REPLACE FUNCTION time_diff (
DATE_1 IN DATE, DATE_2 IN DATE) RETURN NUMBER IS
 
NDATE_1   NUMBER;
NDATE_2   NUMBER;
NSECOND_1 NUMBER(5,0);
NSECOND_2 NUMBER(5,0);
 
BEGIN
  -- Get Julian date number from first date (DATE_1)
  NDATE_1 := TO_NUMBER(TO_CHAR(DATE_1, 'J'));
 
  -- Get Julian date number from second date (DATE_2)
  NDATE_2 := TO_NUMBER(TO_CHAR(DATE_2, 'J'));
 
  -- Get seconds since midnight from first date (DATE_1)
  NSECOND_1 := TO_NUMBER(TO_CHAR(DATE_1, 'SSSSS'));
 
  -- Get seconds since midnight from second date (DATE_2)
  NSECOND_2 := TO_NUMBER(TO_CHAR(DATE_2, 'SSSSS'));
 
  RETURN (((NDATE_2 - NDATE_1) * 86400)+(NSECOND_2 - NSECOND_1));
END time_diff;
/
 
 
// Another method, possibly much faster:
 
SELECT (TO_DATE('01-JAN-16')-TO_DATE('12-DEC-15'))*86400 FROM DUAL;