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 -)  -