16 For this cause the Lord knowing the greatness of the adversary grants indulgence to the godly, saying, Then let them which be in Judæa flee to the mountains. But if any man is conscious that he is very stout-hearted, to encounter Satan, let him stand (for I do not despair of the Church's nerves), and let him say, Who shall separate us from the love of Christ and the rest? But, let those of us who are fearful provide for our own safety; and those who are of a good courage, stand fast: for then shall be great tribulation, such as has not been from the beginning of the world until now, no, nor ever shall be. But thanks be to God who has confined the greatness of that tribulation to a few days; for He says, But for the elect's sake those days shall be shortened; and Antichrist shall reign for three years and a half only.
We speak not from apocryphal books, but from Daniel; for he says, And they shall be given into his hand until a time and times and half a time . A time is the one year in which his coming shall for a while have increase; and the times are the remaining two years of iniquity, making up the sum of the three years; and the half a time is the six months. And again in another place Daniel says the same thing, And he swore by Him that lives for ever that it shall be for a time, and times, and half a time. And some perhaps have referred what follows also to this; namely, a thousand two hundred and ninety days; and this, Blessed is he that endures and comes to the thousand three hundred and five and thirty days. For this cause we must hide ourselves and flee; for perhaps we shall not have gone over the cities of Israel, till the Son of Man be come.
Source: Catechetical Lectures (New Advent)