Here are my recommendations for time synchronizers for PCs (Macs have this functionality built in)… Both update automatically using a frequency you set.

Accurate time on your servers is essential if you are running an Apple Xserve that is bound to your Windows Active Directory (for user authentication) since Apple’s “Active Directory DS plugin” uses Kerberos which is very time sensitive and basically won’t work if the time skew between itself and the AD servers is too large.

1. ArGoSoft Time Synchronizer - freeware
http://www.argosoft.com/rootpages/TimeSync/Default.aspx

Simple with “Synchronize Now” button and settings for update frequency. Settings via a Control Panel.

2. Dimension 4 - free for personal use (donations welcome); US$10 for commercial use
http://www.thinkman.com/dimension4/

Has a lot of features and a nice System Tray icon to either “Synchronize now” or “Open” to change settings.

Screenshots: