4 But as our life here is exposed to numerous and great temptations, and it is to be feared lest we may be turned aside by them from that refuge, let us see what in consequence of this the prayer of the man of God seeks for. “Turn not Thou man to lowness”: that is, let not man, turned aside from Your eternal and sublime things, lust for things of time, savour of earthly things. This prayer is what God has Himself enjoined us, in the Prayer, “Lead us not into temptation,” He adds, “Again You say, Come again, you children of men.” As if he said, I ask of You what You have commanded me to ask: giving glory to His grace, that “he that glories, in the Lord he may glory:” without whose help we cannot by an exertion of our own will overcome the temptations of this life. “Turn not Thou man to lowness: again you say, Turn again, you children of men.” But grant what You have enjoined, by hearing the prayer of him who can at least pray, and aiding the faith of the willing soul.
5. “For a thousand years in Your sight are but as yesterday, which is past by”: hence we ought to turn to Your refuge, where You are without any change, from the fleeting scenes around us; since however long a time may be wished for for this life, “a thousand years in Your sight are but as yesterday:” not as tomorrow, which is to come: for all limited periods of time are reckoned as having already passed. Hence the Apostle's choice is rather to aim at what is before, that is, to desire things eternal, and to forget things behind, by which temporal matters should be understood. But that no one may imagine a thousand years are reckoned by God as one day, as if with God days were so long, when this is only said in contempt of the extent of time: he adds, “and as a watch in the night:” which only lasts three hours. Nevertheless men have ventured to assert their knowledge of times, to the pretenders to which our Lord said, “It is not for you to know the times or seasons, which the Father has put in His own power:” and they allege that this period may be defined six thousand years, as of six days. Nor have they heeded the words, “are but as one day which is past by:” for, when this was uttered, not a thousand years only had passed, and the expression, “as a watch in the night,” ought to have warned them that they might not be deceived by the uncertainty of the seasons: for even if the six first days in which God finished His works seemed to give some plausibility to their opinion, six watches, which amount to eighteen hours, will not consist with that opinion.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)