The classic paper on distributed clocks mentions it, Lamport's "Time, Clocks, and the Ordering of Events in a Distributed System" (http://research.microsoft.co

The classic paper on distributed clocks mentions it, Lamport's "Time, Clocks, and the Ordering of Events in a Distributed System" (http://research.microsoft.com/en-us/um/people/lamport/pubs/time-clocks.pdf http://research.microsoft.com/en-us/um/people/lamport/pubs/t...)