If you assume that the Universe is governed by immutable universal physical laws, then time is the construct that separates past and future.  With our current understanding of thermodynamics, our Universe moves inexorably towards more disorder, or an increase in entropy, and colder temperatures, or a decrease in enthalpy. This is better described as the "Running down of the Stars," where matter and energy become increasingly dissipated throughout space.  This dissipation gives a benchmark for a "beginning time," from which all other events can be measured.  In other words, at the beginning of time, at the moment of the Big Bang, the Universe was small and hot.  In many billions of years, it will transform to large and cold. Contrarily, if the Universe were contracting, time, or the sequence of events would to our understanding appear to run backward -- the Universe would get smaller and hotter. In that Universe, the future would be the past.