15:13:46 sql> 
15:13:46 sql> create table ycz_test_1
2 (row_id varchar2(100),
3 start_dt varchar2(100),
4 end_dt varchar2(100));

table created

executed in 0.02 seconds
15:13:47 sql> create table ycz_test_2
2 (row_id varchar2(100),
3 start_dt date,
4 end_dt date);

table created

executed in 0.03 seconds
15:13:47 sql> insert into ycz_test_1
2 values('1','','');

1 row inserted

executed in 0.01 seconds
15:13:47 sql> commit;

commit complete

executed in 0.01 seconds
15:13:47 sql> insert into ycz_test_2
2 values
3 ('1',to_date('2007-1-1', 'yyyy-mm-dd'), to_date('2008-2-1', 'yyyy-mm-dd'));

1 row inserted

executed in 0.01 seconds
15:13:47 sql> commit;

commit complete

executed in 0.01 seconds
15:13:47 sql> update ycz_test_1 a
2 set (a.start_dt,a.end_dt)=
3 (select b.start_dt,b.end_dt
4 from ycz_test_2 b
5 where a.row_id = b.row_id);

1 row updated

executed in 0 seconds

15:13:47 sql> commit;

commit complete

executed in 0.01 seconds

15:13:54 sql> select * from ycz_test_1;

row_id start_dt end_dt
------------------------
1 01-jul-07 01-aug-08

executed in 0.03 seconds

15:14:02 sql> 
15:18:07 sql> 
15:18:08 sql> update ycz_test_1 a
2 set (a.start_dt,a.end_dt)=
3 (select to_char(b.start_dt,'yyyy-mm-dd'),to_char(b.end_dt,'yyyy-mm-dd')
4 from ycz_test_2 b
5 where a.row_id = b.row_id);

1 row updated

executed in 0.01 seconds

15:18:08 sql> select * from ycz_test_1;

row_id start_dt end_dt
--------------------------------------
 -------------------------------------
 -------------------------------------
1 2007-01-01 2008-02-01

executed in 0.03 seconds