“The concept of one event happening before another in a distributed system is examined, and is shown to define a partial ordering of the events.”
Leslie Lamport, Time, Clocks, and the Ordering of Events in a Distributed System (1978) · Abstract · read it
Kept because it quietly gives up something everyone assumes: that events have an order. In a system with more than one clock, before is only partly defined, and most distributed bugs are someone forgetting that.