Oracle: subtract millisecond from a datetime

I thought it was really simple but it isn't.

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY') 
          - 1/(24*50*60*1000) data 
FROM dual;

It simply doesn't work.

Other details:

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY') -
           NUMTODSINTERVAL(1/(24*50*60*1000),'HOUR') data 
FROM dual;

doesn't work..

The right seems to be

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY') -
           NUMTODSINTERVAL(1/(24*25*60*1000),'HOUR') data 
FROM dual;

Why? How does it work?