Lamport Clocks
Computers generally track the current time using a quartz crystal that oscillates at a known frequency. This frequency is used to advance the local clock at pre-defined intervals. Quartz oscillates with enough stability that it can be used to maintain time within a few milliseconds of accuracy per day. Over time however, these imperfections can accumulate, making the clock inaccurate. To fix these inaccuracies, computers typically run a background process like NTP to synchronize the local clock with servers that are known to be accurate....