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 Thats operational  see Physics Engineering John Doe  I think we should stop here per wpTPG since we are no longer discussing the article but the subject And we are severely repeating ourselves