Since the beginning of physics time is the duration of material changes. We measure time with clocks. The notion of time in Newton physics, Einstein’s relativity and quantum physics are different despite we always measure the same time with the same apparatuses that are clocks. We showed in this article that the act of the measurement done by the observer is generating duration. Time as duration is the result of the interaction between the observer and physical reality via clocks. In the universe, only changes exist. Changes have no duration on their own. Time as duration is born with the measurement done by the observer.