Joseph Smith Testimony, 1:45; “He commenced, and again related the very same things which he had done at his first visit, without the least variation; which having done, he informed me of great judgments which were coming upon the earth, with great desolations by famine, sword, and pestilence; and that these grievous judgments would come on the earth in this generation. Having related these things, he again ascended as he had done before.”
Jeremiah 27:12-15; “I spake also to Zedekiah king of Judah according to all these words, saying, Bring your necks under the yoke of the king of Babylon, and serve him and his people, and live. 13 Why will ye die, thou and thy people, by the sword, by the famine, and by the pestilence, as the LORD hath spoken against the nation that will not serve the king of Babylon? 14 Therefore hearken not unto the words of the prophets that speak unto you, saying, Ye shall not serve the king of Babylon: for they prophesy a lie unto you. 15 For I have not sent them, saith the LORD, yet they prophesy a lie in my name; that I might drive you out, and that ye might perish, ye, and the prophets that prophesy unto you.”
Smith “wrote” this in 1838. We can see this is a false testimony and that he plagiarized part of Jeremiah. It has been 172 years at the time of this writing from the time Smith wrote the prophecy that “this generation will be destroyed”. A generation is typically seen as 40 years.