Have you verified that the time on the DC is accurate and staying accurate? I would have to imagine that windows will have let you know with an event when it updates system time.
Unchecking that box simply means that VMware isn't trying to update the guest with the host's time. It's important to remember that physical devices have a dedicated hardware clock in them. Physical ones are using an emulated clock that is based on CPUcycles and that CPUcycles get skippedas the hostgets busier.
In theory you could point your host to an NTP server and then use the time option to sync your PDC to the host. VMware employs "Smart" clock cyclycingto graduallycatch the clock up or set it back. Transactional applications that are highly sensitive and don't like the clock being set backwards, think banking applications. VMwarewill micro set...