Physical Clock count number of seconds elapsed. This clocks has problem with instrument measurement accuracy. Classical clocks use Quartz clocks with low accuracy (1 ppm = 1 microsecond/second = 86 ms/day = 32 s/year), atomic clocks (1 second in 3 million years) but costs ~ £ 20_000. For synchronizations use NTP Servers.
Logical clock count events, e.g. messages sent (relevant in distributed systems). This clocks requires communication between notes.