For time cannot be know to exist without the now, any more than a line without a point. Thus, every given now has time preceding it and following it, so that no now can be either first of last. It remains that mobile things, which created substances are, exist from eternity. If, therefore, a thing's existence is affirmed as a result of denying it, then that thing must exist always. Now, time is such a thing.
Time is, therefore, necessarily eternal.
But time is an accident, and an accident cannot be without a subject. Now, God, who is above time, is not the subject of this accident, for He is altogether immutable, as we proved in Book I of this work. Furthermore, the fifth argument, drawn from a consideration of time, suppose the eternity of motion rather than proves it. Clearly, then, the same instant is the beginning of the future and the end of the past because some assigned point in motion is the beginning and the end of the diverse parts of motion. On the other hand, if we held that motion is not eternal, we can say that the first instant of time is the beginning of the future and the terminus of no time past.