An Epoch is a Fixed Point in Space and Time.
Epochs are defined as the specific point in space and time.
All other times are relative.
Most calendars on Earth use the Gregorian Calendar Epoch (implemented in Rome 1582, England 1752, and Russia 1918), or the Unix Time Epoch (implemented January 1 1970, corrected through ????).
We have, on occasion, used the Synchronized Convertible Elizabeth II Epoch. This system measures dates from the day of the death of Queen Elizabeth II.