Well, St. Thomas gives me a lesson, and I think it's a great lesson. He answers the question in a way that, while perhaps not as technical as some folks these days might like, still provides just the sort of answer that I need. Time, he says, is the number of motion or change.
For since succession occurs in every movement, and one part comes after another, the fact that we reckon before and after in movement, makes us apprehend time, which is nothing else but the measure of before and after in movement (ST I Q10 A1).This makes sense when thinking of ourselves, but not with respect to God. God doesn't move because he's omnipresent (there is no place where he is not, so it's not possible for him to "go" anywhere), and he does not change.
Thus eternity is known from two sources: first, because what is eternal is interminable--that is, has no beginning nor end (that is, no term either way); secondly, because eternity has no succession, being simultaneously whole (ibid.)Every moment of our existence is one great "now" for God. He knows the end from the beginning precisely because there is no beginning or end for him. Contrast this truth with the error of the so-called "open theists" who deny God's omniscience. Such a god as they imagine must be trapped in time, subject to change like the rest of us. But this is not the God of the Bible, nor yet even the God we may know by reason.
The idea of eternity follows immutability, as the idea of time follows movement, as appears from the preceding article. Hence, as God is supremely immutable, it supremely belongs to Him to be eternal. Nor is He eternal only; but He is His own eternity; whereas, no other being is its own duration, as no other is its own being. Now God is His own uniform being; and hence as He is His own essence, so He is His own eternity (ST I Q10 A2).