Use INTERVAL types : INTERVAL « PL SQL Data Types « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. PL SQL Data Types
3. INTERVAL

SQL>
SQL> SET SERVEROUTPUT ON
SQL>
SQL> DECLARE
  2     v_college_deadline TIMESTAMP;
  3  BEGIN
  4     v_college_deadline := TO_TIMESTAMP('06/06/2004', 'DD/MM/YYYY')
  5                           + INTERVAL '12-3' YEAR TO MONTH;
  6
  7     DBMS_OUTPUT.PUT_LINE('My daughter leaves for college in '
  8                          ||v_college_deadline);
  9  END;
 10  /
My daughter leaves for college in 06-SEP-16 12.00.00.000000 AM

PL/SQL procedure successfully completed.

SQL>
SQL>