can't update timestamp columns

 Hi,

Does anyone know why the following update didn't set the value in the timestamp columns mod_time? It is still null after teh update.

SQL> desc xmm.t_interval;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 INTERVAL_ID                               NOT NULL NUMBER
 MOD_TIME                                           TIMESTAMP(6)

SQL> select INTERVAL_ID from  xmm.t_interval where mod_time is null and rownum < 2;

INTERVAL_ID
-----------
       7904

SQL> alter session set  NLS_DATE_FORMAT='MMDDYYYYHH24MISS';

Session altered.

SQL> update xmm.t_interval set MOD_TIME='04022010151129.398068000' where interval_id = 7904;

1 row updated.

SQL> select mod_time from xmm.t_interval where interval_id = 7904 and mod_time is null;