tardisnt.exe is a shareware program the synchronizes your clock with various time servers. More information can be found here.

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Tardis is a shareware utility for Windows that makes sure your PC?s clock tells the right time. It can find out what the right time is in various ways including accessing Internet-based Atomic Clocks, using networked timeservers, GPS (The Global Positioning System), Radio clocks, and by listening for time broadcasts over a LAN.

The Service version of Tardis 2000 runs as a Windows NT/2000/XP/2003 "service", just like other services that come as standard with Windows.