Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no contradiction between the fact that no person knows when the Last Day will come and exactly knowing some of the things that will occur before it and which Allah made as signs and pre-requisite to the coming of this Day.
Indeed, Allah left the knowledge of the precise date (day-month-year) of this Day to Himself. It is of His Private Knowledge that nobody else knows.
However, Allah has put some signs that should precede This Day. In deed, The Day of Judgement will not come before these signs take place. These include the descending of Prophet I’sa (Jesus) (Alaihi As-Salaam). The evidence for this is the Hadith reported by Al Bukhari narrated Abu Hurairah that the Prophet (Sallallahu Alaihi wa Sallam) said: “…Ibn Maryam (Jesus) is to descend amongst you and rule with justice”.
Sunnah is evidence in Shariah just like the Qur'an. Allah Says (interpretation of meaning) :{…And whatsoever the Messenger (Muhammad SAW) gives you, take it, and whatsoever he forbids you, abstain (from it)} [59:7]. Even in the Qur'an there is a reference to the coming of I’sa (Jesus) (Alaihi As-Salaam) back to earth and that this is a sign of the closeness of the Day of Judgement. Allah Says (interpretation of meaning): {And he ['Iesa (Jesus), son of Maryam (Mary)] shall be a known sign for (the coming of) the Hour (Day of Resurrection) [i.e. 'Iesa's (Jesus) descent on the earth]…} [43:61].
Allah knows best.