BTW - IMO what you posted were not TIMESTAMP, but DATE datatypes.
Code:
1 select to_date('2009-02-15 12:12:12','YYYY-MM-DD HH24:MI:SS') -
2 TO_DATE('1999-04-14 12:12:12','YYYY-MM-DD HH24:MI:SS') INTERVAL
3* FROM DUAL
SQL> /
INTERVAL
----------
3595
difference UNIT of Measure is DAYS