It seems that the server updates its clock from the internet time source once a week. Is it possible to configure the frequency of this update?
The Windows time service is not something I've ever relied upon because the documentation has always stated
If you need to keep your clocks in good sync with a reference clock then you need something that runs the full NTP protocol or similar. http://www.meinberg.de/english/sw/ntp.htm is the NTP implementation I use under Windows (it is a Windows port of the NTPd found in most unix-a-like OSs), though there are others. A proper NTP client will keep your clock in sync with its sources (assuming you have good sources, of course) by slowly skewing the clock to account for inaccuracy instead of changing it in a jump every now and again so your clock should always be accurate (a quick run of
Best practice is to nominate one of your servers as the time keeper and have it run NTPd both as client (to keep its clock in sync with the rest of the world) and server (so your other machines can query it). Pick a set of local servers from the public NTP pool (see http://www.pool.ntp.org/en/) for the timekeeper to sync against. You could sync all your machines to the public pool but this is unfair use of the public resource and means that your clocks will drift apart if something happens to your Internet connectivity or the external pool of timeservers for a time (with the local timekeeper option, your machines will at least all keep in sync with each other during the outage). For the sake of redundancy it is probably legitimate use of the pool to sync two machines with the it, so your local network has a second source to use if one goes down, though I've never felt the need to do this myself.
For example at the office I usually spend my working day in, the little box that acts as firewall and router between the LAN and the outside world runs the standard NTPd found in most Linux setups syncs its clock with the servers
One further note: you need to take a little extra care if you are using Virtual Machines for anything, as keeping their clocks in sync can have extra complications.