In this document “Epoch” is a point in time. It is given as a decimal year in the Gregorian calendar, with yyyy.00 being midnight at the start of the 1st January of year “yyyy”. Should it be required to convert the epoch to or from a Gregorian calendar date, assuming any of 365, 365.25 or 366 days as the year length will be satisfactory for dealing with tectonic plate linear motion.