7 “Days upon days of the King You shall add to the years of Him”. This is therefore the King of whom we are the members. A King Christ is, our Head, our King. You have given to Him days upon days; not only those days in that time that has end, but days upon those days without end. “I will dwell,” he says, “in the house of the Lord, for length of days.” Wherefore for length of days, but because now is the shortness of days? For everything which has an end, is short: but of this King are days upon days, so that not only while these days pass away, Christ reigns in His Church, but the Saints shall reign together with Him in those days which have no end....For years of God have been also spoken of: “But You are the very Same, and Your years shall not fail.” In the same manner as years, so days, so one day.
Whatsoever you will you say of eternity. Whatever you will you say for this reason, because whatever you shall have said, it is too little that you have said. For you must needs say somewhat, to the end that there may be something whereby you may meditate on that which cannot be told. “Even unto the day of generation and of generation.” Of this generation and of the generation that shall be: of this generation which is compared to the moon, because as the moon is new, waxes, is full, wanes, and vanishes, so are these mortal generations; and of the generation wherein we are born anew by rising again, and shall abide for everlasting with God, when now no longer we are like the moon, but like that of which says the Lord, “Then the righteous shall shine like the sun in the kingdom of their Father.” For the moon by a figure in the Scriptures is put for the mutability of this mortal state....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)