A time-line clock glues together a sequence of physical clocks to provide a consistent, clear semantics for a discrete time-line. A time-line clock provides a clear, consistent semantics for a discrete time-line by gluing together a sequence of physical clocks. Since the range of most physical clocks is limited, a time-line clock is usually composed of many physical clocks. For instance, a tree-ring clock can only be used to date past events, and the atomic clock can only be used to date events since the 1950s.