7 “I have thought on ancient days”. Now he, as if he were one who had been beaten out of doors, has taken refuge within: he is conversing in the secret place of his own heart. And let him declare to us what he is doing there. It is well with him. Observe what things he is thinking of, I pray you. He is within, in his own house he is thinking of ancient days. No one says to him, you have spoken ill: no one says to him, you have spoken much: no one says to him, you have thought perversely.
Thus may it be well with him, may God aid him: let him think of the ancient days, and let him tell us what he has done in his very inner chamber, whereunto he has arrived, over what he has leaped, where he has abode. “I have thought on ancient days; and of eternal years I have been mindful.” What are eternal years? It is a mighty thought. See whether this thought requires anything but great silence. Apart from all noise without, from all tumult of things human let him remain quiet within, that would think of those eternal years.
Are the years wherein we are eternal, or those wherein our ancestors have been, or those wherein our posterity are to be? Far be it that they should be esteemed eternal. For what part of these years does remain? Behold we speak and say, “in this year:” and what have we got of this year, save the one day wherein we are. For the former days of this year have already gone by, and are not to be had; but the future days have not yet come. In one day we are, and we say, in this year: nay rather say thou, today, if you desire to speak of anything present.
For of the whole year what have you got that is present? Whatsoever thereof is past, is no longer; whatsoever thereof is future, is not yet: how then, “this year”? Amend the expression: say, today. You speak truth, henceforth I will say, “today.” Again observe this too, how today the morning hours have already past, the future hours have not yet come. This too therefore amend: say, in this hour. And of this hour what have you got? Some moments thereof have already gone by, those that are future have not yet come.
Say, in this moment. In what moment? While I am uttering syllables, if I shall speak two syllables, the latter does not sound until the former has gone by: in a word, in that same one syllable, if it chance to have two letters, the latter letter does not sound, until the former has gone by. What then have we got of these years? These years are changeable: the eternal years must be thought on, years that stand, that are not made up of days that come and depart; years whereof in another place the Scripture says to God, “But You are the Self-same, and Your years shall not fail.” On these years this man that leaps over, not in babbling without, but in silence has thought.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)