Time feels absolute on one machine. In a distributed system, it is not. Every machine has its own clock, those clocks drift apart, and synchronizing them happens over a network with variable delay.

That uncertainty matters because timestamps end up driving correctness decisions: which write happened last, whether a lock has expired, whether a token is still valid, what order to replay events in. Wall-clock time is still useful, but it is the wrong tool when correctness depends on exact ordering across machines.

This chapter covers clock drift, wall vs monotonic clocks, NTP and PTP, where skew causes bugs, and when physical time is the right tool.