SQL> -- this is what you do - add 1 to a string '16-kol-07'

SQL> select to_date('16-kol-07' + 1, 'dd-mon-rr') result from dual;
select to_date('16-kol-07' + 1, 'dd-mon-rr') result from dual
               *
ERROR at line 1:
ORA-01722: invalid number


SQL> -- this is how it should be done - convert string to a date (using TO_DATE) and then add 1

SQL> select to_date('16-kol-07', 'dd-mon-rr') + 1 result from dual;

RESULT
--------
17.08.07

SQL>