Stopwatch

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Stopwatch [http://expect.nist.gov/stopwatch] is a pleasant Tk script that enables hand timing much like a physical stopwatch. It can also be used as a countdown timer. 
Stopwatch has millisecond precision, syncs with the system clock for millisecond accuracy, and has no artificial limit on hours - for practical purposes, its limit is 25 years. Stopwatch runs on UNIX, Windows, and MacOS. 

[http://expect.nist.gov/stopwatch]