6 “The Lord has said unto me, You are My Son, today have I begotten You”. Although that day may also seem to be prophetically spoken of, on which Jesus Christ was born according to the flesh; and in eternity there is nothing past as if it had ceased to be, nor future as if it were not yet, but present only, since whatever is eternal, always is; yet as “today” intimates presentiality, a divine interpretation is given to that expression, “Today have I begotten You,” whereby the uncorrupt and Catholic faith proclaims the eternal generation of the power and Wisdom of God, who is the Only-begotten Son.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)