Where does PostgreSQL get the date/time from?

Hi

Where does PostgreSQL get the date/time from, because it certainly
isn't from my system clock.

If I change the system time, it doesn't change in Postgresql.

It looks like postmaster gets the date and time when you start
up the server and then keeps its own time from then on.

All the computers on our network update their time from the domain
controller, but then the time giving by CURRENT_TIMESTAMP isn't
the same as the time on the computer running the database.

Is there a way to fix this?

Thanks
Colin