Oracle 21c

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create table your_table (start_date date);
insert into your_table
select date '2000-08-11' from dual union all
select cast(timestamp '2008-11-15 00:40:04' as date) from dual union all
select cast(timestamp '2008-11-15 00:19:56' as date) from dual union all
select cast(timestamp '2016-04-29 23:42:02' as date) from dual;
4 rows affected

alter session set nls_date_format = 'DD-MON-YYYY HH24:MI:SS';
select start_date,
  to_char(start_date, 'DD-MON-YYYY HH24:MI:SS') as str,
  dump(start_date, 16) as dmp
from your_table;


START_DATE
STR
DMP


11-AUG-2000 00:00:00
11-AUG-2000 00:00:00
Typ=12 Len=7: 78,64,8,b,1,1,1

15-NOV-2008 00:40:04
15-NOV-2008 00:40:04
Typ=12 Len=7: 78,6c,b,f,1,29,5

15-NOV-2008 00:19:56
15-NOV-2008 00:19:56
Typ=12 Len=7: 78,6c,b,f,1,14,39

29-APR-2016 23:42:02
29-APR-2016 23:42:02
Typ=12 Len=7: 78,74,4,1d,18,2b,3


declare
  d date;
begin
  dbms_stats.convert_raw_value('786c0b0f0115fd', d);
  update your_table set start_date = d
  where start_date = cast(timestamp '2008-11-15 00:19:56' as date);
end;
/
1 rows affected

select start_date,
  to_char(start_date, 'DD-MON-YYYY HH24:MI:SS') as str,
  dump(start_date, 16) as dmp
from your_table;


START_DATE
STR
DMP


11-AUG-2000 00:00:00
11-AUG-2000 00:00:00
Typ=12 Len=7: 78,64,8,b,1,1,1

15-NOV-2008 00:40:04
15-NOV-2008 00:40:04
Typ=12 Len=7: 78,6c,b,f,1,29,5

15-NOV-2008 00:20:04
00-000-0000 00:00:00
Typ=12 Len=7: 78,6c,b,f,1,15,fd

29-APR-2016 23:42:02
29-APR-2016 23:42:02
Typ=12 Len=7: 78,74,4,1d,18,2b,3

START_DATE	STR	DMP
11-AUG-2000 00:00:00	11-AUG-2000 00:00:00	Typ=12 Len=7: 78,64,8,b,1,1,1
15-NOV-2008 00:40:04	15-NOV-2008 00:40:04	Typ=12 Len=7: 78,6c,b,f,1,29,5
15-NOV-2008 00:19:56	15-NOV-2008 00:19:56	Typ=12 Len=7: 78,6c,b,f,1,14,39
29-APR-2016 23:42:02	29-APR-2016 23:42:02	Typ=12 Len=7: 78,74,4,1d,18,2b,3