Only God knows
﴾They say: When will this promise (come to pass) if ye are telling the truth? Say: The appointment to you is for a Day, which ye cannot put back for an hour nor put forward.﴿ (Saba’: 29 and 30)
God will gather all the people on the Doomsday and he will judge them for what they have done. Because people are always looking for excuses to let go of their responsibilities, they keep asking about the time of the Doomsday.
The holy Qur’an never gives a clear answer to this question and emphasizes on the fact that this knowledge belongs to God only.
Keeping this a secret, even from Prophet Mohammad, is because God wants people to be prepared for death all the time; because if people knew about the exact day of the Doomsday, they would act ignorantly and carelessly if it were far away and they might not act based on their free will if it were close. None of these two is what Islam is after.
﴾Verily the Hour is coming - My design is to keep it hidden - for every soul to receive its reward by the measure of its Endeavour.﴿ (Ta Ha: 15)
More than that, you cannot just deny something only because you do not know about its exact details.
God has kept hidden from people some specific dates; for example, people neither know when they will die, nor do they know when they will be rewarded or punished.
Prophet Mohammad said, “There are five things one should appreciate; his youth before he gets old, his wealth before he gets poor, his peace before he gets miserable, and his life before he dies.”
Imam Sadiq said, “If one knows the exact time of his death, he will be sunk into the joys of life and lust, with the hope that he will have time to repent in his last days. This won’t please God and He won’t accept this. This is why God does not want people to know how long they will live, so that they expect death all the time, as long as they are alive; and to turn their backs to the evil deeds as a result of this.”
* Translated by: Sadroddin Musawi