In this context, we must highlight that the definition and understanding of the nature of time is difficult, because any process is usually described from a time viewpoint. In Newtonian physics, time is a mathematical variable, but, it isn't something real . Simultaneity and duration of phenomena are absolute: a duration is an abstract property of the whole. But, Einstein introduced a completely new approach to time in Relativity, where instants and durations are dependent on the observer. The physical universe is spacetime, a mathematical space continuously filled with ideal clocks . These clocks are all synchronized with respect to a given observer, that measures the duration of a phenomenon through two clocks located in the places where the phenomenon starts and ends. So, another inertial observer, in motion with respect to the first one, doesn't agree on the space-time coordinates of the same events; consequently, the same phenomenon has a different duration in relation to any observer. As a consequence of this distinction, the requirement of the definition of physical time emerges . Following Bell and Pauli , Brown pointed out that clocks don't measure time, but their behaviour in relation to some aspects of space-time . Consequently, physical systems evolve in the phase space following the path which increases entropy! But, all these results require a physical definition of time.