A physical clock is a physical process coupled with a method of measuring that process to record the passage of time. For instance, the rotation of the Earth measured in solar days is a physical clock. Most physical clocks are based on cyclic processes (such as a celestial rotation). One or more physical clocks are used to establish a time-line clock for a temporal database.
Every concrete time is a measurement of some physical clock. For instance a windup watch provides the time “now” by measuring the rate at which a coiled, wound spring unwinds. A physical clock is limited by the durability and regularity of the underlying physical process that it measures, so to provide a clock for every instance in a time line for a temporal database, several physical clocks might be needed. For instance, a clock based on the rotation of the Earth will (likely) be limited as current models predict the Earth will eventually be gravitationally drawn into the Sun....