How to modify a external talbe column type

its a external table and I am using this statment to create  the table...
SQL> create table Tier_Pricing_xtern
       ( comid_id NUMBER(10,0),
          Dealer NUMBER(15,5),
         Comm_Vendor NUMBER(15,5),
        T1 NUMBER(15,5),
       T2 NUMBER(15,5),
        T3 NUMBER(15,5),
  T4 NUMBER(15,5),
  S_Dt DATE,
  E_Dt DATE  
      )
       organization external
       ( default directory xtern_Tierdata_dir
         access parameters
         ( records delimited by newline
           fields terminated by ','
         )
         location ('Tier_Pricing.csv')  
     );
The date in the csv looks like this...
100,135.58,119.63,102,88,66,57,3/15/2009,3/22/2009
The log file created shows this....
error processing column S_DT in row 1 for datafile E:\SHILOH\IMPORTS\TierP\Tier_Pricing.csv
ORA-01722: invalid number
 
When I first created the table the date columns were created in number format. The table now shows the date fomat for those columns.... not sure what I did wrong here...

Now I am getting this error...
error processing column S_DT in row 1 for datafile E:\SHILOH\IMPORTS\TierP\Tier_Pricing.csv
ORA-01861: literal does not match format string

thanks for you help... I now have the external table created... check out my new question..
https://www.experts-exchange.com/questions/24232908/Move-data-from-one-external-table-to-two-tables-conditionally.html