Without going into any great depth we might say that time is associated with the change in substance and accidents that created beings undergo, in that it gives a measure of successiveness. This gives us the notions of “before”, “now” and “after”. Time is God’s way of stopping everything from happening at once as far as created beings are concerned. On the other hand, God, being pure act undergoes no change and therefore it makes no sense whatsoever to place Him in time, or even commensurate with time. Rather He is in eternity; we may even say that he is His own eternity.
However, there is also a half-way house between time and eternity: if eternity is associated with changelessness and time is associated with substantial and accidental change, we might inquire into what is associated with those created things that undergo only accidental change (and no substantial change) like the angels and (arguably) like the blessed in heaven after the general resurrection. This is aeviternity.
In Ia 10 a. 5 ad. 2 Aquinas states the quite astonishing fact that:
Aeviternity is simultaneously whole; yet it is not eternity, because "before" and "after" are compatible with it.
When we come finally to rest in Him, when we have attained our finality in the beatific vision, there is no substantial change in us. All that which perfects us in what we are will be present to us. Heaven will not be boring because it will not be one long endless round of harp playing!