In the theory of relativity in the study of physics, the interval between two events is called proper time. Proper time is based on a clock that measures both events.
Proper time is measured by a clock that creates the same point of reference, and is used to compare the two events based on their length. For one common example of measuring proper time, consider a spaceship leaving Earth with one person on it and one person left behind, measuring how long it takes. The spaceship goes to a planet five light years away. The person on Earth measures a time that reads five light years, and the person on the spaceship measures a time that is much shorter. The time the person on the spaceship measures is the proper time, because he was present at both the start and finish.