I run various production systems in both VMware ESX and VirtualBox, and I have time issues in all VMs, regardless of hypervisor vendor. The VM tools are installed and operating "properly." The host machine clocks are synced correctly with ntp. Yet the VMs will sometimes get tens of minutes out of alignment in short order.

At this point I'm thinking I need to run ntp on the physical hardware with the least jitter, then just run ntpdate in a cron job on all the VMs. This would work better than ntp clients in the VMs or the VM tools.