Oracle 11g Release 2

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create table my_table( date_time timestamp );
insert into my_table 
select to_timestamp('20-OCT-2019 13:39:51.654321', 'DD-MON-YYYY HH24:MI:SS.FF')  
  from dual;
1 rows affected

select * from my_table;

DATE_TIME

20-OCT-19 01.39.51.654321 PM

update my_table t
   set t.date_time = t.date_time + interval '-3' day;
   
1 rows affected

select * from my_table;

DATE_TIME

17-OCT-19 01.39.51.654321 PM