71 But perhaps it may be held to confirm the Son in His confession of ignorance that He says the Father alone knows. But unless He had plainly said that the Father alone knows, it would have been a matter of the greatest danger for our understanding, since we might have thought that He Himself did not know. For, since His ignorance is due to the economy of hidden knowledge, and not to a nature capable of ignorance, now that He says the Father alone knows, we cannot believe that He does not know; for, as we said above, God's knowledge is not the discovery of what He did not know, but its declaration.
The fact that the Father alone knows, is no proof that the Son is ignorant: He says that He does not know, that others may not know: that the Father alone knows, to show that He Himself also knows. If we say that God came to know the love of Abraham, when He ceased to conceal His knowledge, it follows that only because He did not conceal it from the Son, can the Father be said to know the day, for God does not learn by sudden perception, but declares His knowledge with the occasion. If, then, the Son according to the mystery does not know the day, that He may not reveal it: on the other hand, only by the fact that He has revealed it can the Father be proved to know the day.
Source: On the Trinity (New Advent)