AWS Developer Forums: Clock synchronization in EC2 ...
The host machine should be syncronizing the clock. Virtual machines should then simply see a common clock shared by the machine. In Xen this would be under dom0 while instances run under domU(s) which do not have access to the hardware clock. I have heard of a patch that allows clock shifting where Xen maintains a differential between the hardware clock on your domU VM. But I would imagine that patch would slow down access to the clock by excluding it from the hardware VM assist features (which otherwise allow every VM to read the clock instantly w/o a hypervisor trap happening).So the answer would be: just don't run NTP (except to provide time server feed to your own LAN). Hopefully each box at AWS has NTP running in their dom0.
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