score:23
Prophecy about Jesus' return within 56 years
Per the History of the Church, vol. 2, p. 189, he said
...and go forth to prune the vineyard for the last time, or the coming of the Lord, which was nigh--even fifty-six years should wind up the scene.
That was in 1891.
Prophecy that the temple would be built in Missouri within Smith's Generation
_Doctrines and Covenants 84:2-5,31.
For verily this generation shall not all pass away until an house shall be built unto the Lord, and a cloud shall rest upon it, which cloud shall be even the glory of the Lord, which shall fill the house.
All Nations would be involved in the American Civil War war
_Doctrine and Covenants 87:1-3
For behold, the Southern States shall be divided against the Northern States, and the Southern States will call on other nations, even the nation of Great Britain, as it is called, and they shall also call upon other nations, in order to defend themselves against other nations; and then war shall be poured out upon all nations.
That short list came from here.
Some more include:
etc. That list is here.
A note to all would-be prophets... don't use exact dates.