What does “Epoch” and “Epoch clock” mean?

Unix time (also known as POSIX time or epoch time) is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970.

For purposes of control at specific times it allows  long analog numbers to be effectively used in control logic to represent start and end times without having to write calendar functions.