ABSTRACT
People could use any calendar as a clock, laying it down that every reign was to count as an equal length of time, in fact they have a rationale for their system for measuring time, and believe that it is not just their fiat which deems periods to be isochronous. Essentially, people adopt some principle of time-invariance; they talk about time timelessly, so that they can talk about intervals at different times being the same, just as they can talk about colours, lengths or fields at different times being the same. Some clocks, the water clock and King Alfred's candles, are linear processes; with them we measure time simply by the volume of water that has flowed or the length of candle consumed. The principle of time-invariance required for the rational theory of clocks is that of the date-indifference of processes. The principle of date-indifference can also be expressed as the principle of the causal irrelevance of time.
