For the last year+ I've been dealing with a maddening issue. All the machines in my house were always off by 3-8 minutes from the atomic clock. Not that big of a deal for most folks... but when you've got Windows Media Centers with the wrong time, you always have your recordings start a few minutes late.
The way I've got my house setup is that I have a virtualization rig (Win2k8 with Hyper-V) that hosts my two domain controllers (all machines at home are on the same domain except a little Windows Home Server) and a bunch of others for work stuff. The host OS on the virtualization rig is joined to the domain. The primary DC is setup to sync it's time with one of the many available time servers.
Was driving me nuts that no matter how many times I'd reset the time on the DC, within a few minutes/hours it would fall behind again. I lived with this frustration, and the negative looks from my wife when one of her/our shows didn't start recording on time, for months. Finally the fix hit me!
Hyper-V's integration services has one service that sync's the time in the VM's with that of the host of the host OS. Hyper-V was making the changes! I went into the settings for all VMs on my network that were joined to the domain (as the domain sets the time of all the joined machines) and disabled the option that syc'd the time. Woke up the next morning and all the machines on my network matched the time shown on our cell phones (the ATandT network) which also matched the atomic clock and the clock in the DirecTV guide!