Objection 1. It would seem that the creation of things was not in the beginning of time. For whatever is not in time, is not of any part of time. But the creation of things was not in time; for by the creation the substance of things was brought into being; and time does not measure the substance of things, and especially of incorporeal things. Therefore creation was not in the beginning of time.
Objection 2. Further, the Philosopher proves (Phys. vi, text 40) that everything which is made, was being made; and so to be made implies a "before" and "after." But in the beginning of time, since it is indivisible, there is no "before" and "after." Therefore, since to be created is a kind of "being made," it appears that things were not created in the beginning of time.
Objection 3. Further, even time itself is created. But time cannot be created in the beginning of time, since time is divisible, and the beginning of time is indivisible. Therefore, the creation of things was not in the beginning of time.
On the contrary, It is said (Genesis 1:1): "In the beginning God created heaven and earth."
I answer that, The words of Genesis, "In the beginning God created heaven and earth," are expounded in a threefold sense in order to exclude three errors. For some said that the world always was, and that time had no beginning; and to exclude this the words "In the beginning" are expounded--viz. "of time." And some said that there are two principles of creation, one of good things and the other of evil things, against which "In the beginning" is expounded--"in the Son." For as the efficient principle is appropriated to the Father by reason of power, so the exemplar principle is appropriated to the Son by reason of wisdom, in order that, as it is said (Psalm 103:24), "Thou hast made all things in wisdom," it may be understood that God made all things in the beginning--that is, in the Son; according to the word of the Apostle (Colossians 1:16), "In Him"--viz. the Son--"were created all things." But others said that corporeal things were created by God through the medium of spiritual creation; and to exclude this it is expounded thus: "In the beginning"--i.e. before all things--"God created heaven and earth." For four things are stated to be created together--viz. the empyrean heaven, corporeal matter, by which is meant the earth, time, and the angelic nature.
Reply to Objection 1. Things are said to be created in the beginning of time, not as if the beginning of time were a measure of creation, but because together with time heaven and earth were created.
Reply to Objection 2. This saying of the Philosopher is understood "of being made" by means of movement, or as the term of movement. Because, since in every movement there is "before" and "after," before any one point in a given movement--that is, whilst anything is in the process of being moved and made, there is a "before" and also an "after," because what is in the beginning of movement or in its term is not in "being moved." But creation is neither movement nor the term of movement, as was said above (45, 2,3). Hence a thing is created in such a way that it was not being created before.
Reply to Objection 3. Nothing is made except as it exists. But nothing exists of time except "now." Hence time cannot be made except according to some "now"; not because in the first "now" is time, but because from it time begins.
Source: Summa Theologica (New Advent)