linux "date -s" command not working to change date on a server

http://www.linuxforum.com/threads/2154-Linux-ntp-time-Offset-on-Xen-VM-incorrect

After a lot of searching I found that by default, the VM's clocks are synchronized to the HOST clock running on the control domain, and cannot be independently changed. This was surprising to me because NTP was still configured and appeared able to synchronize the clocks.

add the following line to your /etc/sysctl.conf file

# Allow the VM to update it's own clock, and do not use the DOM host clock.
xen.independent_wallclock=1

then restart the network service

/etc/init.d/network restart #for redhat, centos, fedora
/etc/init.d/networking restart #debian, ubuntu

now you can set the date with date -s ...

Here is a link to the official xen docs
Here is a link to the ntp docs