An update of
http://www.redhat.com/archives/libvir-list/2010-February/msg00104.html
The two current options are:
<clock offset='utc'/>
<clock offset='localtime'/>
This introduces a way to set a specific timezone
<clock offset='timezone' timezone='Europe/Paris'/>
And a way to set a completely arbitrary time, by giving the number
of seconds offset from UTC.
<clock offset='variable' adjustment='123456'/>
Since last time, I removed the 'timezone' attribute from the existing
offset='localtime' option, and introduced the new explicit option
offset='timezone'. That way we don't change semantics of the existing
options at all.