But he probably does not need to be so precise, right ? He could create
a 'timestamp' column to replace the 'date' one, and accept to lose the
last second for the previous data, calculating the number through
something like this:

SQL> select time_diff('01-JAN-70',sysdate) * 1000 from dual;

TIME_DIFF('01-JAN-70',SYSDATE)
------------------------------
1147182144000

(code for time_diff : http://www.psoug.org/reference/date_func.html)