We associate an event with a number on a clock We define that number as the time of the event according to that clock We measure the time of the start event and the ending event of a process We define the absolute value of the difference between the numbers as the time the process takes according to that clock See also Metrology That s operational see Physics Engineering John Doe I think we should stop here per wp TPG since we are no longer discussing the article but the subject And we are severely repeating ourselves