create table test
(a date)
/

SCOTT @ORCL> select * from test
  2  /

A
-------------------
28/08/2008 05:18:19

:global.g_sys_time := null;
declare
	my_timer timer; 
begin 
	--Create a timer to trigger it every 1 sec(1000msec) 
	my_timer := create_timer('timer_sysdate',1000,repeat); 
end; 

:global.g_sys_time := to_char(sysdate,'hh:mi:ss');

declare
	v_date_time varchar2(30);
begin

	select to_char(nvl(:test.a, sysdate), 'dd/mm/rrrr')||' '||:global.g_sys_time
	into v_date_time
	from dual;
	
	select to_date(v_date_time, 'dd/mm/rrrr hh:mi:ss')
	into :test.a
	from dual;

end;

declare
	v_date_time varchar2(30);
begin

	select to_char(nvl(:test.a, sysdate), 'dd/mm/rrrr')
	into v_date_time
	from dual;
	v_date_time := v_date_time||' '||to_char(sysdate,'hh:mi:ss');

	select to_date(v_date_time, 'dd/mm/rrrr hh:mi:ss')
	into :test.a
	from dual;

end;