Most of these responses seem to be attempts at physics based explanations so here is a philosophical perspective: Aristotle defined time as the measurement of motion with respect to the before and after. In this sense time is a numerical value that describes how far apart the beginning and end of a motion (or more generally any change) are. As distance is the measurement of a given length, so time is a measurement of a motion across that length not counting the length itself. 🙂
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