It seems there are some issues with xen hwclock and centos and ubuntu xen vms !

• http://howtoforge.com/debian_sarge_xen_3.0.3_p7
• https://help.ubuntu.com/community/Xen
• http://wiki.xensource.com/xenwiki/CoolConfigurations
• http://jailtime.org/download:centos:v5.1?s=hwclock

You may find that installing
# yum install system-config-date
and using timeconfig instead might help