How to log out users on a Windows server 2008 domain at a certain time

I have been trying to get a Windows domain to autologoff users at 6:00pm each day.  After reading some docs online I found one possible way to do it but it does not seem to work. I set the valid logon time of the users to stop at 6:00pm and in the default domain policy under the computers section in the local policies section the option to auto logoff users when their logon time expires. I refreshed the policy on the server and workstation but does not autologoff. If I try logging in after 6:00 it does deny me logon but does not auto logoff if I am already logged in.  Does anyone have any ideas how to make this work?
Thanks,

Jeff