Aquinas states this, which is interesting, that God is Eternity. These ideas are found in the First Part, Question 10 of the Summa.
Eternity is nothing else but God Himself. Hence God is not called eternal, as if He were in any way measured; but the idea of measurement is there taken according to the apprehension of our mind alone.
... Words denoting different times are applied to God, because His eternity includes all times; not as if He Himself were altered through present, past and future.
Obviously, being people of time, we cannot understand easily a God Who is totally outside of time and Who Is "Permanent Being". And, even though we have these beautiful ideas, I think that the best way to try and understand the Attributes, as much as we are able to do so, is through active contemplation.
If God so blesses us with infused knowledge, as He has granted to those who allowed themselves to be purged of sin and egotism, then such ideas will become not only clearer, but experiential to a certain degree.
Eternity is simultaneously whole. But time has a "before" and an "after." Thereforetime and eternity are not the same thing.
I answer that, It is manifest that time and eternity are not the same. Some have founded this difference on the fact that eternity has neither beginning nor an end; whereas time has a beginning and an end. This, however, makes a merely accidental, and not an absolute difference because, granted that time always was and always will be, according to the idea of those who think the movement of the heavens goes on for ever, there would yet remain a difference between eternity and time, as Boethiussays (De Consol. v), arising from the fact that eternity is simultaneously whole; which cannot be applied to time: for eternity is the measure of a permanent being; while time is a measure of movement. Supposing, however, that the aforesaid difference be considered on the part of the things measured, and not as regards the measures, then there is some reason for it, inasmuch as that alone is measured by time which has beginning and end in time. Hence, if the movement of the heavens lasted always, time would not be of its measure as regards the whole of its duration, since the infiniteis not measurable; but it would be the measure of that part of its revolution which has beginning and end in time.
Another reason for the same can be taken from these measures in themselves, if we consider the end and the beginning as potentialities; because, granted also that time always goes on, yet it is possible to note in time both the beginning and the end, by considering its parts: thus we speak of the beginning and the end of a day or of a year; which cannot be applied to eternity. Still these differences follow upon the essential and primary differences, that eternity is simultaneously whole, but that timeis not so.