The day of judgement will never be appear before the arrival of Imam
Prophet Muhammad (saww) said:
"Even if the entire duration of the world's existence has already been exhausted and only one day is left before the Day of Judgment, Allah will expand that day to such a length of time, as to accommodate the kingdom of a person out of Ahl al-Bayt who will be called by my name and my father's name. He will then fill the Earth with peace and justice as it will have been filled with injustice and tyranny before then."
[Sahih Tirmidhi, V2, P86, V9, P74–75]
Imam al-Mahdi had already born just a few years ago(ie, the day of
judgement nears us)
Twelver Shia Muslims believe that Imam Mahdi _ may Allah hasten his advent_ is the Twelfth Imam, who was born in 869 and was hidden by God at the age of five (874). He is still alive but has been in occultation, "awaiting the time that God has decreed for his return".
Even Imam al-Mahdi does not know he is Imam al-Mahdi.
Since he is the twelfth Imam and has been awaiting his advent for hundreds of years, of course he knows this. however, it's true that even Imam Mahdi (aj) doesn't know when his advent is.
At the time of His Hajj, a sound from sky will appear which sounds
that he is Imam al-Mahdi.(This will happen within a two or three
- There are certain specific signs about Imam Mahdi(aj)'s advent, one of them is the Cry/Scream of supernatural origin, coming from the sky and calling man to defend the Imam’s cause. This call is said to be by Gabriel.[Birth and occultation of the Mahdi]
- The ultimate arrival of Imam Mahdi_ may Allah hasten his advent_ is unknown and only Allah knows when his advent is.
Muslims all over the world (Sunnis, Shias, and all other Muslims) will
come under his Khalifath.
When Imam Mahdi (aj) comes, the whole world will be taken to submission. Imam Mahdi (aj) will be in control upon the earth for certain time, after which ensues the death of all civilization just preceding the Day of Judgment.
Imam Ja'far al-Sadiq (as), the Sixth Imam, has made the following prophecy:
"The Mahdi will conquer the world; at that time the world will be illuminated by the light of God, and everywhere in which those other than God are worshiped will become places where God is worshiped; and even if the polytheists do not wish it, the only faith on that day will be the religion of God."
[Bihar al-Anwar: 51: 146]
Conquering the world is predicted in Quran and this will happen to be true by Imam Mahdi a.s.
هُوَ الَّذِي أَرْسَلَ رَسُولَهُ بِالْهُدَى وَدِينِ الْحَقِّ
لِيُظْهِرَهُ عَلَى الدِّينِ كُلِّهِ وَلَوْ كَرِهَ الْمُشْرِكُونَ
It is He who sent His Messenger with guidance and the religion of
truth to manifest it over all religion, although those who associate
others with Allah dislike it.
Then only Dhajjal will come.
Masih ad-Dajjal is an evil figure in Islamic eschatology. He is to appear pretending to be Hazrat Masih (as). Shias believe that Dajjal will be killed by Imam Mahdi (aj).[source]
The following QA is from the book "پرسشهای شما و پاسخهای آیت الله العظمی بهجت" written by Hamed Islamju, published with a permission from Ayatullah Bahjat's office, page 50.
Q: Some people say that even Imam Mahdi himself doesn't know the time for his rise, is it right?
A: in a Hadeeth from Imam Sadiq --peace be upon him-- he says "…. we wait and endure what we know but our Shia followers wait and endure what thy don't know". How patient is Imam Mahdi! He knows all what we know and what we don't know, he is aware of all our troubles and difficulties. He is waiting for the day to rise as well but he knows when it would be. That is sometimes stated that he doesn't know its time is not correct.
This also has many witnesses in Ahadeeth when Imams say they are told by the holy prophet --peace be upon him and his household-- every happening that will occur until the Day of Judgement! That they know Sufyani, his name and his fathers' names, his apparent looking and many more. After all the prophet and his household are those firmly grounded in knowledge cited in Quran.
In the book "عصر شکوهمند رجعت" (The glorious era of Return) written by Muhammad Reza Akbari, many different Ahadeeth on Return (Rij'ah) is stated and discussed, it is actually a research book on the issue. After a beautiful discussion on great many Ahadeeth it states that one can classify the Ahadeeth about what happens after Imam's passing away in 6 categories:
- Twelve Mahdi becoming rulers
- Jesus being a ruler
- fifty years of chaos
- forty days being left to the Judgement day
- Imam Hussein becoming a ruler (whose ruling would last for 300 years and nine years more) and after him Imam Ali (a moving creature from the Earth according to Quran: دابة الأرض, see [27:82]) becoming a ruler
The three first categories are least probable as the Ahadeeth pointing toward them are rather weak, so the 4th and 5th category are more probable, and the fifth one is most probable. Maybe one wants to gather these two categories in one category by stating that those two imam would return in the time of ruling of Imam Mahdi, like the other Imams and all the prophets would return in his time, but it is not probable when we mention they will have their own rulings.
Anyway, after the last friend of Allah passes away, be it Imam Mahdi or Imam Ali, peace be upon them, there would be forty days left to the end of this world. People of this time are the worst of ever and the news about the wrath of Allah over them is spread everywhere in Quran, people are always scared of such a wrath, cf. [22:2].