[fedora-virt] Windows clock synchronization

Hello, I have clock synchronization problems with Fedora 18/KVM host and windows server guests. Different 64bit windows servers, all has some time synchronization problems. Some of them will slow down time until time difference is 60 seconds and then clock is synchronizes from internet. But 60 seconds is a long time. Kerberos authentication sometimes files due to time skew. Some machines will run OK day or two, but then clock stops. After some hours clock is synchronized again and continues to work properly. Curious, that during time is stopped, server works properly (except kerberos authentications, which is very time sensitive. For example, attaching 2 graphs from 2 windows servers. They show time difference measured by check_mk nagios plugin. 10k means 10000 seconds. How I should set clock/time/NTP on these guests? I have absolutelly no problems with Linux guests, they work perfectly. Thank you. SAL