This interaction of Mass and Energy results in Movement. Movement produces change and it is the change recognized by an observer that gives Time it's conception. For each observer in a given inertial frame the rate of change is observed as constant.
Movement, then, is also continuous in the
way in which time is - indeed time is either
identical to movement or is some affection of it.
(Aristotle, Metaphysics)
The rate of change can vary in a given inertial frame but since the change affects the whole frame of the observer it can not be detected. This change therefore appears constant for a given observer’s locale. An observer can compare frames and detect variations in the rate of change relative to each. This gives the illusion that Time slows down or speeds up.
We percieve time passing because we remember when things were different and we call it the past. We think of the future because we can anticipate how things will be different. The observer recognizing change brought about by movement is only possible because of memory. Reason can anticipate change and the mind imagines the future. This all happens in the present now. The Now is held together by God who upholds the physical reality that creates the illusion of time by the observation of change brought on by movement that is a result of the interaction of Mass and Energy.
Without the mind's memory and capacity to reason time has no meaning. The illusion of time helps us make sense of reality. It brings cohesion to existence and our place in all that exists.