You might also like "AboutTime.exe". It can be configured for multiple time sources which it tries in order until it gets a response. It also provides a local time SERVER that you can use to feed time to other local PCs. It can be configured to update at just about any interval you like, it minimizes to the system tray, etc.
My experience with the windows time service is terrible. None of the systems in my office use it, it's the 'pits.' My HTPC is on my home WAN and is configured to make a VPN connection to a server in my office so that it can do periodic backups of configuration data, databases, etc. I found that with windows time service running the VPN was constantly busy and in a 12 hour period it moved 100MB of "garbage" data over the VPN for no apparent reason --- and it never did set the time properly :-) I disabled it and used AboutTime and the problem went away.
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