"Not a valid month" when getting a count by comparing timestamps

I am trying to see how many items are older then 3 years. When I use

select count (*) from ae_dt509 where field13 < to_timestamp( '2013-06-15 00:00:00', 'YY-MM-DD HH:MI:SS' )

I get this error:

Error starting at line 1 in command:
select count (*) from ae_dt509 where field13 < to_timestamp( '2013-06-12 00:00:00', 'YY-MM-DD HH:MI:SS' )
Error report:
SQL Error: ORA-01843: not a valid month
01843. 00000 -  "not a valid month"
*Cause:    
*Action:

I am not sure whats wrong...

EDIT

I changed the code to correct the year:

select count (*) from ae_dt509 where field13 < to_timestamp( '2013-06-15 00:00:00', 'YYYY-MM-DD HH24:MI:SS' )

and I got the same error.

and then changed it by removing the qoutes around the date:

select count (*) from ae_dt509 where field13 < to_timestamp( 2013-06-15 00:00:00, 'YYYY-MM-DD HH24:MI:SS' )

and got

Bind Variable "00" is NOT DECLARED