:udatetime:= to_date(to_char(SYSDATE,'hh24:mi:ss'),'hh24:mi:ss');

Yes Sir you are right ! You can see my code which i posted on 15-NOV i used in pre-insert trigger which is successfully working but @tara_260985 requirement seems little different that's why i have had decided to make one database trigger.

------------------------Save current datetime example----

Create table xyz(udatetime date);

BLOCK-LEVEL
  >>>>>PRE-INSERT 

Insert into xyz (udatetime) select Sysdate from dual;

SQL>  Select to_Char(udatetime,'DD-MM-YYYY HH24:MI:SS') from xyz;

TO_CHAR(UDATETIME,'
-------------------

15-11-2014 07:30:55

15-11-2014 07:31:23

or 

:udatetime:= to_date(to_char(SYSDATE,'hh24:mi:ss'),'hh24:mi:ss');

may be your scenario could be different in that case let me know. I given you just example.

Hope i will help you.

Yes Sir you are right ! You can see my code which i posted on 15-NOV i used in pre-insert trigger which is successfully working but @tara_260985 requirement seems little different that's why i have had decided to make one database trigger. Trigger syntax something like that

------------------------Save current datetime example----
Create table xyz(udatetime date);

BLOCK-LEVEL
  >>>>>PRE-INSERT 

Insert into xyz (udatetime) select Sysdate from dual;

SQL>  Select to_Char(udatetime,'DD-MM-YYYY HH24:MI:SS') from xyz;

TO_CHAR(UDATETIME,'
-------------------

15-11-2014 07:30:55

15-11-2014 07:31:23

---------Syntax---Trigger Detail-------

CREATE [OR REPLACE ] TRIGGER trigger_name
  {BEFORE | AFTER | INSTEAD OF } 
  {INSERT [OR] | UPDATE [OR] | DELETE} 
  [OF col_name]  
  ON table_name
  [REFERENCING OLD AS o NEW AS n]
  [FOR EACH ROW]  WHEN (condition)  
 BEGIN 
   --- sql statements 
 END; 

My database table my_table already consists of these two columns ,but I don't want to display in the layout editor.