Most of those slides concern about the physics part of time measurement (GPS and atomic clock, etc.). While this is interesting in its own right, in order to understand how MY computer obtains the current time, a more relevant question is “how does a home computer measure the latency of a packet sent from a remote time server”? Does it measure the durations of several roundtrips and take the average duration as latency? What if congestion suddenly occurs during some roundtrip? I always think that these questions are more mysterious than the physical ones.