Are your clients all windows based?
When the workstation authenticates with the domain controller its time is automatically synched. If this is just a workgroup then time will have to be set manually. You can use the net time console command to set the time of the server and or workstations to a ntp timer server.
http://www.windowsnetworking.com/nt/.../atips47.shtml
I do not know if it is possible to sync a linux client with windows server. AFAIK for syncing linux clients you need to install the ntp package.