SQL & PL/SQL

Hi ,
I am runing the following statment and get result back as follow:

SELECT d.table_name, d.partition_name, d.tablespace_name,
  2                 to_date(SUBSTR (d.partition_name,
  3                         2,
  4                         LENGTH (d.partition_name) - 5
  5                         )
  6                 || SUBSTR (d.partition_name,
  7                     LENGTH (d.partition_name) - 3,
  8                     LENGTH (d.partition_name)
  9                     ) ,'YYYYMMDD') part_date
 10                 FROM user_tab_partitions d
 11                 WHERE d.table_name = 'RADIUS_LOG' AND d.partition_name NOT LIKE '%MAX%';

TABLE_NAME                     PARTITION_NAME                 TABLESPACE_NAME                PART_DATE
------------------------------ ------------------------------ ------------------------------ ---------
RADIUS_LOG                     P20091111                      RADIUS_DATA_TS                 11-NOV-09
RADIUS_LOG                     P20091112                      RADIUS_DATA_TS                 12-NOV-09
RADIUS_LOG                     P20091113                      RADIUS_DATA_TS                 13-NOV-09

After adding the part_date to the where clause i am getting : ORA-01830: date format picture .
Why ?

 select t.table_name,t.partition_name, t.tablespace_name,to_date(t.part_date,'dd/mm/rrrr') p_date
  2    from(
  3    SELECT d.table_name, d.partition_name, d.tablespace_name,
  4                  to_date(SUBSTR (d.partition_name,
  5                          2,
  6                          LENGTH (d.partition_name) - 5
  7                          )
  8                  || SUBSTR (d.partition_name,
  9                      LENGTH (d.partition_name) - 3,
 10                      LENGTH (d.partition_name)
 11                      ) ,'YYYYMMDD') part_date
 12                  FROM user_tab_partitions d
 13                  WHERE d.table_name = 'RADIUS_LOG' AND d.partition_name NOT LIKE '%MAX%'
 14         ) t 
 15     where to_date(t.part_date,'dd/mm/rrrr') < to_date(sysdate,'dd/mm/rrrr') - 7;
                || SUBSTR (d.partition_name,
                 *
ERROR at line 8:
ORA-01830: date format picture ends before converting entire input strin

Thanks