This subject has been discussed a million times, and I have spent a few hours reading one site after the next and I think a need new eyes to have a look at this at tell me what to do.
On many sites (http://blogs.msdn.com/b/virtual_pc_guy/archive/2010/11/19/time-synchronization-in-hyper-v.aspx
seems to be the most "official"), it is recommended to NOT disable the Time Sychronization.
One has to disable in the guest VM, on the domain controller (sbs 2011 in my case), that the sychronization is made with the parent host (which is a hyper-v core, not connected to the internet, not in the domain).
I did so and if I query the ntp source, I have pool.ntp.org, which is correct.
If I force a sync, the time is set accurately, but a few seconds later, it jumps again a few minutes (4 in my case). This case is also mentioned on many site (and on the link above).
Please help, this is really driving me nuts :)