SQL> alter session set nls_Date_format='HH24:MI:SS';

Session altered.

SQL> select * from p;

        ID A          D
---------- ---------- --------
         1 test       21:05:19

SQL> host cat t_test.sql
CREATE OR REPLACE TRIGGER d_trig
BEFORE UPDATE ON p
FOR EACH ROW
BEGIN
        select sysdate into :new.d from dual;
END d_trig;
/

SQL> @t_test

Trigger created.

SQL> update p set a='foo';

1 row updated.

SQL> select * from p;

        ID A          D
---------- ---------- --------
         1 foo        21:08:43

SQL> commit;

Commit complete.

SQL>