Generally, when Time is discussed in philosophy or in science — including the works of classical Jewish philosophers — it is understood as movement, or change. The movement of the planets, the ticking of a clock — as long as these things occur, there is Time. If they cease, Time also ceases. In other words, Time is no more than the occurrence of physical events.
In this paradigm, once we accept the creation of heaven and earth ex nihilo, it is a simple conclusion that Time began then as well — since Time is no more than the events of the cosmos. The fact that we cannot fathom an absence of Time is irrelevant. After all, neither can we fathom an absence of heaven and earth — in other words, of space and matter. In this way, Maimonides and most of the classic Jewish philosophers determine, contrary to the opinion of Aristotle, that Time is a creation with a beginning.
(from philosophic essay)