When working with Virtual domaincontrollers on Hyper-V you are very likely to have come across the time drifting problem.
I have found a very nice site which gives so good insight in the workings of Hyper-V Time Synchronisation and also why you should keep it enabled.
http://blogs.msdn.com/b/virtual_pc_guy/archive/2010/11/19/time-synchronization-in-hyper-v.aspx
Ofcourse it is always recommended that you keep atleast one domain controller running on a non-virtual machine but in some cases this is not possible.
The common problem I have ran into is that running a VM on a domain-joined DC starts giving problems. Once again you should ofcourse not join the Hyper-V host to the domain but in smaller deployments such “luxerier” can sometimes not be afforded. Work-around is to not have the clock sync to the external time source (use the registery value) and use something such as Keir’s Neutron to manually sync time on the domain controller every 15 / 30 minutes.