Why would anyone pray earnestly to know the time of the 2nd coming when Jesus said even he didn't know---seems like that is wanting to know more than Jesus does. If God chose to not tell Jesus, who is any man that He should divulge that to?
"15 Joseph, my son, if thou livest until thou art eighty-five years old, thou shalt see the face of the Son of Man; therefore let this suffice, and trouble me no more on this matter."
As someone else already mentioned, God's prophecies are never dependent on the shortcomings of one single man. Stephen was stoned, didn't keep things from happening as God intended. At the same time, some of God's prophecies are dependent on the faithfulness of a people, or some other act. God does then say "if"--clearly, there is an "if" here.
The question asked of God was as to the time of His coming. So the answer has to be dependent on that "if" qualifier, which is JS living to 85. So what this is saying is that if JS lives to 85, he will see the face of God--I am sorry but that makes no sense at all. It doesn't answer the question asked. It doesn't even say that JS would see the 2nd coming, just that he would see the face of God. This in no way means that He will see him when JS is 85- but means he has to live to 85--sometime in his 85th year he would see His face. It still is not a date for the 2nd coming.
What really doesn't make any sense either is that JS has to be alive past 85 to see His face---no matter what age we are at our death--at the resurrection everyone that is saved will see the face of God at His 2nd coming. Everyone. Those that do not see that 2nd coming are the lost---the living lost are destroyed at His 2nd coming, and the dead wicked remain dead until after the 1000 years. Technically, if this prophecy were of God, this could easily be interpreted to mean that if JS doesn't live to 85--he will never see the face of God at that 2nd coming, because he will be amongst the lost. And, according to JS--he had already seen the face of God in his visions.
If this were of God it also means that Mormonism would have had to go throughout all the world by JS's 85th birthday. Mat_24:14 And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
The end will come when God sees fit--it is not dependent on any one individual living to a certain age. God has that event already set even before Jesus came into this world.
2Pe_3:9 The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
God is always right on time. Basically, this is a nonsense prediction of nothing.