Including one of the classics of computer science, of course:<p><i>Time, Clocks and the Ordering of Events in a Distributed System</i><p><a href="http://research.microsoft.com/en-us/um/people/lamport/pubs/pubs.html#time-clocks" rel="nofollow">http://research.microsoft.com/en-us/um/people/lamport/pubs/p...</a>