Now we beseech you, brethren, by the coming of our Lord Jesus Christ and by our gathering together unto Him, that ye be not soon shaken in mind or be troubled, neither by spirit, nor by word, nor by letter as if from us that the Day of Christ is at hand. (2 Thessalonians 2:1-2)
OK, so if an epistle says that the Day of Christ is at hand, it isn't really from Paul.
Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And so shall we ever be with the Lord. Therefore comfort one another with these words. But as to the times and the seasons, brethren, ye have no need that I write unto you. For you yourselves know perfectly that the Day of the Lord so cometh as a thief in the night.
(1 Thessalonians 4:17-18, 5:1-2)
Whoa! 1 Thessalonians just said that the end will come suddenly during Paul's lifetime. So, if 2 Thessalonians is telling us the truth, that makes 1 Thessalonians a liar!