ANSWER:

The default service in Windows which typically synchronises time is the W32Time service, whose configuration parameters are situated in the registry at HKLMSYSTEMCurrentControlSetServicesW32Time. That will give you some insight into how time synchronisation is taking place if the native Windows mechanism is in use.

Additionally, you can also look for events in the System event log – typically around system start – which will be logged if and when the system has synchronised with some external time source using the W32Time provider (other software tools may or may not have the same mechanism for logging).

There appears to be some confusion over terminology, though. Machines do not “synchronise” with their BIOS. The BIOS may represent a (local) source of time for a machine which otherwise does not have an upstream server to synchronise with. In particular, note that the domain controller in the forest root domain in possession of the PDC emulator operations role considers itself the most authoritative time source for the whole enterprise; it is effectively the clock to which all other machines should be synchronised for authentication protocols like Kerberos to work effectively, and whether you choose to synchronise its clock with an external time source to ensure its accuracy is entirely a personal choice (the environment will work perfectly fine if the time is not accurate).

tigermatt from http://serverfault.com/questions/509238