A simple tip for keeping up with the time in Xen when your domU isn't syncing the clock frequently enough and your clock is whatever. Of course you can sync your clock with ntpdate but first you need to set the domain (domU) to run its wallclock independently from Xen.
Simply say in the command line of domU:
echo 1 > /proc/sys/xen/independent_wallclock
Or if you have noclobber on:
echo 1 >! /proc/sys/xen/independent_wallclock
To reenable tracking of Xen wallclock:
echo 0 > /proc/sys/xen/independent_wallclock
And to keep the setting between reboots, just add it to the /etc/sysctl.conf:
xen.independent_wallclock = 1
Although, it is better if the Xen dom0 syncs the clock frequently so all the domains (domU) don't need to do it by themselves and thus wasting resources. Why do things multiple times when it could be done just once.