ORA-01843: not a valid month

BEGIN
          SELECT  emp_id,
                  initials,
                  dob,
                  first_name,
                  middle_initial,
                  last_name,
            INTO  g_emp_id,
                  g_initials,
                  g_dob,
                  g_first_name,
                  g_middle_initial,
            FROM  emp
    
 
          dbms_output.put_line(new_emp.dob);
 
          IF to_date(g_dob,'MM/DD/YYYY') <> to_date(new_emp.dob,'MM/DD/YYYY') OR 
             g_initials <> new_emp.emp_initials OR 
          THEN
            BEGIN
 
            
              UPDATE  emps
                SET   dob = to_date(new_emp.dob,'MM/DD/YYYY'),
                      initials = new_emp.emp_initials,
                WHERE emp_id = g_emp_id;
               
            EXCEPTION
              WHEN OTHERS THEN
                g_msg_string := 'Updating emp record.' || CHR(10) ||
                                     sqlerrm || CHR(10);
            END;
          END IF;
 
 
                             scott.UPDATE_emp ( g_emp_id, 
                                                new_emp.first_name, 
                                                new_emp.middle_initial, 
                                                new_emp.last_name, 
                                                '', 
                                                '', 
                                                '', 
                                                g_err_nbr, 
                                                g_err_msg );
 
          IF g_err_msg IS NOT NULL 
          THEN
                g_msg_string := 'Updating emp record.' || CHR(10) ||
                                     g_err_msg || CHR(10);
          END IF;
 
        COMMIT;
 
        EXCEPTION 
        WHEN NO_DATA_FOUND
        THEN
 
        --
        -- emp does not exist in TW. Create using both site Id and enrollment id.
        --
 
         g_msg_string := 'Inserting emp record.' || CHR(10);
 
                  INSERT INTO emps
                                     (
                                      dob,
                                      first_name,
                                      middle_initial,
                                      last_name,
                                      )
                               VALUES             
                                     (
                                      TO_DATE(new_emp.dob, 'DD-MON-RR'), 
                                      new_emp.FIRST_NAME    ,
                                      new_emp.MIDDLE_INITIAL    ,
                                      new_emp.LAST_NAME    ,
                                     )
                          RETURNING emp_id INTO g_emp_id;
 
 
        COMMIT;
 
        END;