Joseph Smith seemed confident in his prophecies. From Jeff Lindsay:
In February 1844, when persecution in Illinois had become severe once again, Joseph “prophesied that within five years we should be out of the power of our old enemies, whether they were apostates or of the world; and told the brethren to record it, that when it comes to pass they need not say they had forgotten the saying.” This is recorded in History of the Church , Vol. 6, p. 225. By 1849, the Saints were gathered in Utah (the first wave entered the Salt Lake area in July of 1847) and had indeed escaped the power of their old enemies.– Jeff Lindsay – Mormon Answers: Fulfilled Prophecies of Joseph Smith
Why would Joseph Smith have specified the number of years? Why not just say they would escape their enemies?