I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him. And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed.
-Book of Daniel 7:13-14
Almighty God is celebrated as "Ancient of days" because He is of all things both Age and Time,--and before Days, and before Age and Time. And yet we must affirm that He is Time and Day, and appointed Time, and Age, in a sense befitting God, as being throughout every movement unchangeable and unmoved, and in His ever moving remaining in Himself, and as being Author of Age and Time and Days. Wherefore, in the sacred Divine manifestations of the mystic visions, He is represented as both old and young; the former indeed signifying the "Ancient" and being from the beginning, and the latter His never growing old; or both teaching that He advances through all things from beginning to end,----or as our Divine initiator says, "since each manifests the priority of God, the Elder having the first place in Time, but the Younger the priority in number; because the unit, and things near the unit, are nearer the beginning than numbers further advanced. ...Almighty God we ought to celebrate, both as eternity and time, as Author of every time and eternity, and "Ancient of days," as before time, and above time; and as changing appointed seasons and times; and again as being before ages, in so far as He is both before eternity and above eternity and His kingdom, a kingdom of all the Ages. Amen.
-St. Dionysius the Areopagite
"And came to the Ancient of days." By the Ancient of days he means none other than the Lord and God and Ruler of all, and even of Christ Himself, who maketh the days old, and yet becometh not old Himself by times and days. "His dominion is an everlasting dominion." The Father, having put all things in subjection to His own Son, both things in heaven and things on earth, showed Him forth by all as the first-begotten of God, in order that, along with the Father, He might be approved the Son of God before angels, and be manifested as the Lord also of angels."
-St. Hippolytus
