If the PDC emulator master in this forest is not configured to correctly synchronize time from a valid time source, it might ...

If the PDC emulator master in this forest is not configured to correctly synchronize time from a valid time source, it might use its internal clock for time synchronization. If the PDC emulator master in this forest fails or otherwise becomes unavailable (and if you have not configured a reliable time server (GTIMESERV) in the forest root domain), other member computers and domain controllers in the forest will not be able to synchronize their time.