How to select the millisecond/microsecond parts of the datetime c olumn

Hello,

Product version: Postgres 6.5.3

SELECT DATETIME('MILLISECOND', 'NOW'::DATETIME) always returns 0(ZERO) and
so does 'MICROSECOND'. However a reading of the manuals indicate that the
database supports much higher precision values. How do I retrieve the
additional precision values (beyond seconds) when I use the datetime field?
This is a requirement for a conversion project that I am working on. Could
someone help please.

Thanks
Kannan