Of God and His Creatures

St Thomas could scarcely accept the whole account, given by the opponent, of an 'incomplete volition,' notably the statement that a volition is incomplete, "when there is expected the arrival of some opportune time that is not yet come": otherwise, antecedently to creation, God's volition of creating would be incomplete. St Thomas's use of 'at that time' (tunc), speaking of creation, has this difficulty, that time began only with creation. There is nothing to mark creation starting at one point of time rather than at another, looking at the eternal now of God. We can only measure the date of creation backwards, and say that infinite time has not elapsed since creation; and that doubtless is what St Thomas meant, as his next answer shows.

Of God and His Creatures: 2.32