Mufathel asked Imam Al Saadeq pbuh: "Is there for the Ma'moor(the who who is commanded)Awaited Mahdi pbuh a time whom the people know?So he said pbuh:Allah hasn't willed to give a time of his appearance,during a time when our Shias will know,I said: O Master,Why is that? He said pbuh: BECAUSE HE IS THE HOUR that Allah the Most High said about:"They ask you about the Hour when will it Rise?Say,the knowledge is with my Lord- it will not manifest for its time except He;it became heavy in the heavens and the earth" AlAraaf:187
And it is the Hour that Allah said about:"They ask you about the Hour when will it Rise?",And said:"And with Him is the knowledge of the Hour",And he didn't say it is with anyone. AlNazi'at:42
And he said:"Do they wait for the Hour to come to them by surprise for its conditions have come" Luqman:34
And he said:"The Hour came near and the moon has been split" Qamar:1
Besharat Al-Islam p.352
Now read this:
Messenger of Allah s.a.w.w. said: The Hour will not rise until the Qaem of the truth rises.
Bihar AlAnwar v.51 p.65
A Qaem who comes before a Qaem and that is Ahmed.