What exactly is time? Well surprisingly time is much harder to define than one would imagine without there being a circular reference to time itself. Here is a definition from Wikipedia:
Time is a dimension in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them…Some simple, relatively uncontroversial definitions of time include “time is what clocks measure” and “time is what keeps everything from happening at once”
So at the heart of the answer is the ordering of events which prevents everything from happening at once. Sounds very much like a discrete event simulation. It is this thinking that has some scientists looking to answer the question whether we ourselves exist in some computer simulation.