I have heard different explanations but none so clear. This fits quite well with experience. Some jobs seem to drag on forever but in retrospect those days just blew bye. The Greeks (maybe Aristotle) defined time as "the mental measurement of the duration of change." This all makes time changing with speeds close to the speed of light or with very large masses even more confusing as time seems to be our construct. I know there's more to it than that but people smarter than me have been confused by it.