Objection 1. It would seem that time was not created simultaneously with formless matter. For Augustine says (Confess. xii, 12): "I find two things that Thou didst create before time was, the primary corporeal matter, and the angelic nature." Therefore time was not created with formless matter.
Objection 2. Further, time is divided by day and night. But in the beginning there was neither day nor night, for these began when "God divided the light from the darkness." Therefore in the beginning time was not.
Objection 3. Further, time is the measure of the firmament's movement; and the firmament is said to have been made on the second day. Therefore in the beginning time was not.
Objection 4. Further, movement precedes time, and therefore should be reckoned among the first things created, rather than time.
Objection 5. Further, as time is the extrinsic measure of created things, so is place. Place, then, as truly as time, must be reckoned among the things first created.
On the contrary, Augustine says (Gen. ad lit. i, 3): "Both spiritual and corporeal creatures were created at the beginning of time."
I answer that, It is commonly said that the first things created were these four--the angelic nature, the empyrean heaven, formless corporeal matter, and time. It must be observed, however, that this is not the opinion of Augustine. For he (Confess. xii, 12) specifies only two things as first created--the angelic nature and corporeal matter--making no mention of the empyrean heaven. But these two, namely, the angelic nature and formless matter, precede the formation, by nature only, and not by duration; and therefore, as they precede formation, so do they precede movement and time. Time, therefore, cannot be included among them. But the enumeration above given is that of other holy writers, who hold that the formlessness of matter preceded by duration its form, and this view postulates the existence of time as the measure of duration: for otherwise there would be no such measure.
Reply to Objection 1. The teaching of Augustine rests on the opinion that the angelic nature and formless matter precede time by origin or nature.
Reply to Objection 2. As in the opinion of some holy writers matter was in some measure formless before it received its full form, so time was in a manner formless before it was fully formed and distinguished into day and night.
Reply to Objection 3. If the movement of the firmament did not begin immediately from the beginning, then the time that preceded was the measure, not of the firmament's movement, but of the first movement of whatsoever kind. For it is accidental to time to be the measure of the firmament's movement, in so far as this is the first movement. But if the first movement was another than this, time would have been its measure, for everything is measured by the first of its kind. And it must be granted that forthwith from the beginning, there was movement of some kind, at least in the succession of concepts and affections in the angelic mind: while movement without time cannot be conceived, since time is nothing else than "the measure of priority and succession in movement."
Reply to Objection 4. Among the first created things are to be reckoned those which have a general relationship to things. And, therefore, among these time must be included, as having the nature of a common measure; but not movement, which is related only to the movable subject.
Reply to Objection 5. Place is implied as existing in the empyrean heaven, this being the boundary of the universe. And since place has reference to things permanent, it was created at once in its totality. But time, as not being permanent, was created in its beginning: even as actually we cannot lay hold of any part of time save the "now."
Source: Summa Theologica (New Advent)