Physical clocks can be imprecise for the sake of total ordering across multiple processes

Time, Clocks, and the Ordering of Events in a Distributed System 1978

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A system is distributed if the message transmission delay is non negligable relative to the time between events in a single process

Lamport's Logical Clocks

Abstractly, a clock doesn't need to be physical. We represent it as C(x) which is a function that assigns a number to an event, x

  1. each process Pi must increment Ci between any successive events
  2. if event a is a message sent by Pi with a timestamp = Ci(a), then recieving event b on process Pj must set its clock Cj greater than Ci(a)