Sorry, seems I misunderstood the question before.
In case the guest OS's time differs after vMotion, there might be an issue with the ESXi host's time synchronization. Can you confirm that all hosts as well as the DC synchronize their time from a physical time server, i.e. not from each other?
What may also be wort looking at - if there's no time difference between the hosts - is http://kb.vmware.com/kb/1189.
André