Objection 1. It would seem that this time is not hidden. Because when we know exactly the beginning of a thing, we can know its end exactly, since "all things are measured by a certain period" (De Generat. ii). Now the beginning of the world is known exactly. Therefore its end can also be known exactly. But this will be the time of the resurrection and judgment. Therefore that time is not hidden.
Objection 2. Further, it is stated (Apocalypse 12:6) that "the woman who represents the Church had a place prepared by God, that there she might feed [Vulgate: 'they should feed her'] a thousand two hundred sixty days." Again (Daniel 12:11), a certain fixed number of days is mentioned, which apparently signify years, according to Ezekiel 4:6: "A day for a year, yea a day for a year I have appointed to thee." Therefore the time of the end of the world and of the resurrection can be known exactly from Holy Writ.
Objection 3. Further, the state of the New Testament was foreshadowed in the Old Testament. Now we know exactly the time wherein the state of the Old Testament endured. Therefore we can also know exactly the time wherein the state of the New Testament will endure. But the state of the New Testament will last to the end of the world, wherefore it is said (Matthew 28:20): "Behold I am with you... to the consummation of the world." Therefore the time of the end of the world and of the resurrection can be known exactly.
On the contrary, That which is unknown to the angels will be much more unknown to men: because those things to which men attain by natural reason are much more clearly and certainly known to the angels by their natural knowledge. Moreover revelations are not made to men save by means of the angels as Dionysius asserts (Coel. Hier. iv). Now the angels have no exact knowledge of that time, as appears from Matthew 24:36: "Of that day and hour no one knoweth, no not the angels of heaven." Therefore that time is hidden from men.
Further, the apostles were more cognizant of God's secrets than others who followed them, because they had "the first-fruits of the spirit" (Romans 8:23)--" before others in point of time and more abundantly," as a gloss observes. And yet when they questioned our Lord about this very matter, He answered them (Acts 1:7): "It is not for you to know the times or moments which the Father hath put in His own power." Much more, therefore, is it hidden from others.
I answer that, As Augustine says (Qq. lxxxiii, qu. 58) "as to the last age of the human race, which begins from our Lord's coming and lasts until the end of the world, it is uncertain of how many generations it will consist: even so old age, which is man's last age, has no fixed time according to the measure of the other ages, since sometimes alone it lasts as long a time as all the others." The reason of this is because the exact length of future time cannot be known except either by revelation or by natural reason: and the time until the resurrection cannot be reckoned by natural reason, because the resurrection and the end of the heavenly movement will be simultaneous as stated above (Article 1). And all things that are foreseen by natural reason to happen at a fixed time are reckoned by movement: and it is impossible from the movement of the heaven to reckon its end, for since it is circular, it is for this very reason able by its nature to endure for ever: and consequently the time between this and the resurrection cannot be reckoned by natural reason. Again it cannot be known by revelation, so that all may be on the watch and ready to meet Christ: and for this reason when the apostles asked Him about this, Christ answered (Acts 1:7): "It is not for you to know the times or moments which the Father hath put in His own power," whereby, as Augustine says (De Civ. Dei xviii, 53): "He scatters the fingers of all calculators and bids them be still." For what He refused to tell the apostles, He will not reveal to others: wherefore all those who have been misled to reckon the aforesaid time have so far proved to be untruthful; for some, as Augustine says (De Civ. Dei xviii, 53), stated that from our Lord's Ascension to His last coming 400 years would elapse, others 500, others 1,000. The falseness of these calculators is evident, as will likewise be the falseness of those who even now cease not to calculate.
Reply to Objection 1. When we know a thing's beginning and also its end it follows that its measure is known to us: wherefore if we know the beginning of a thing the duration of which is measured by the movement of the heaven, we are able to know its end, since the movement of heaven is known to us. But the measure of the duration of the heavenly movement is God's ordinance alone, which is unknown to us. Wherefore however much we may know its beginning, we are unable to know its end.
Reply to Objection 2. The thousand two hundred sixty days mentioned in the Apocalypse (12:6) denote all the time during which the Church endures, and not any definite number of years. The reason whereof is because the preaching of Christ on which the Church is built lasted three years and a half, which time contains almost an equal number of days as the aforesaid number. Again the number of days appointed by Daniel does not refer to a number of years to elapse before the end of the world or until the preaching of Antichrist, but to the time of Antichrist's preaching and the duration of his persecution.
Reply to Objection 3. Although the state of the New Testament in general is foreshadowed by the state of the Old Testament it does not follow that individuals correspond to individuals: especially since all the figures of the Old Testament were fulfilled in Christ. Hence Augustine (De Civ. Dei xviii, 52) answers certain persons who wished to liken the number of persecutions suffered by the Church to the number of the plagues of Egypt, in these words: "I do not think that the occurrences in Egypt were in their signification prophetic of these persecutions, although those who think so have shown nicety and ingenuity in adapting them severally the one to the other, not indeed by a prophetic spirit, but by the guess-work of the human mind, which sometimes reaches the truth and sometimes not." The same remarks would seem applicable to the statements of Abbot Joachim, who by means of such conjectures about the future foretold some things that were true, and in others was deceived.
Source: Summa Theologica (New Advent)