SQL & PL/SQL

when i am executing the following queries

select trunc('25-JUL-08','YEAR') from dual;

select trunc('25-JUL08','MONTH') from dual ;

none of the query is working in sql developer and prompting the following message

Error starting at line 1 in command:

select trunc('25-JUL-08','MONTH')

from DUAL

Error report:

SQL Error: ORA-01722: invalid number

01722. 00000 -  "invalid number"

*Cause:   

*Action: