ORA-01843: not a valid month

SQL> insert into rousrs values ('ABHAT','TMPSTG','2013-02-05 00:00:00');
insert into rousrs values ('ABHAT','TMPLSTG','2013-02-05 00:00:00')

ERROR at line 1:
ORA-01843: not a valid month

                                             
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';

insert into rousrs values ('ABHAT','TMPSTG','2013-02-05 00:00:00');

1 row inserted.