2. Change the NLS_DATE_FORMAT of your session. If you are using windows, and if I'm not mistaking, you will have to operate this change in the registry. I don't think the following will work on Windows (ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MM-YYYY'), but I need to check this out.
SQL> alter session set nls_date_format = 'YYYY/MM/DD';
SQL> SELECT SYSDATE FROM DUAL;
So, my guess is you should be able to change the NLS_DATE_FORMAT in your session without any problems. If you see that your ALTER SESSION... has no impact, than check out if there is any value set in the registry (LOCAL_MACHINE/SOFTWARE/ORACLE...)