About the“signs of Reappearance” , and the fact that most of the signs have already occurred and only a few remain to take effect, we have explained that the signs of reappearance are only groundwork and not the absolute reason for reappearance. Occurrence of these signs does not make certain the matter of reappearance. Traditions on“Ba’da” (change of Divine decision) testify that the signs of reappearance are all worthy of change, and what is indispensable is the very essence of reappearance of Hazrat Mahdi (a.t.f.s.). One matter in which Imam Mahdi (a.t.f.s.) bears resemblance to Hazrat Joseph is the following:
“Allah would correct the task of his reappearance (‘Zuhoor’) in a night’s time.” 
Besides, numerous traditions say that:
“Those who fix the time of Reappearance are liars and have uttered lies”
Imam’s Reappearance is likened to the setting up of the judgement Day. Just as the time of judgement Day is unclear, the time of Imam’s reappearance too is not known to anyone.
Moreover, the immaculate Imams (a.s.) have stressed that no time has been fixed for reappearance and Faraj and so, we too cannot fix any time.
From the traditions we understand that the Almighty Allah hasn’t fixed or decreed the time of reappearance. Its knowledge is only with Allah, and He has not conveyed this even to the holy Prophet and Imams. Thus, if someone says that Hazrat would reappear during such and such year, it would imply that until that year, Hazrat would certainly not reappear. This itself is an open lie, and against the aforesaid traditions; as our traditions teach us that:
“Await the reappearance of your master every day and night.” 
Another result that would arise by fixing time of reappearance is that its occurrence later than the fixed time is impossible. This too has been rejected by traditions.
Therefore, those who fix the time of reappearance are liars because he will reappear whenever Allah wishes.