Imam Rabbani reveals that the Mahdi will appear in the second millennium following the death of our Prophet (saas):
"It is expected that, after 1,000 years, this secret administration will be renovated. It will be granted with superiority... This way, genuine virtue will emerge and cover the degradation... And it is expected that the Mahdi, the supporter, will come. May God be pleased with him." (Imam Rabbani, Letters of Rabbani, vol. 1, p. 569)
The confirmation of the religious law, the renovation of people takes place in this second thousand. The just witnesses of the truthfulness of this cause are the existence of the Prophet Isa [as] and Mahdi in this period (the second thousand years). (Imam Rabbani, Letters of Rabbani, vol. 1, p. 611)
Those among the nation of Allah's Messenger (saas) are highly perfected in faith. It does not matter whether they are few in number, even a thousand years passes after the death of our Prophet (saas). Their being perfected in faith serves to the following end: strenghtening the Islamic law in its true sense.The coming of the Mahdi after 1,000 years serves this purpose. Our Prophet (saas), the last Messenger, gave the glad tidings of his coming. The Prophet ‘Isa (as), on the other hand, will come down after 1,000 years. (Imam Rabbani, Letters of Rabbani, vol. 1, p. 440)
A thousand years after the death of our Prophet (saas), people will enter the second millennium. According to Imam Rabbani, quoted above, the Mahdi and the Prophet ‘Isa (as) will come during the time between this millenium and the second millenium.
Great Islamic scholar Imam as-Suyuti states in the hadiths of our Prophet (saas) that the life span of the community will be limited to 1500 years:
The life of this community will exceed 1,000 years, yet be limited to 1,500 years. (Muhammad ibn `Abd ar-Rasul Barzanji, Al-Isha`ah li Ashrat as-Sa`ah, p. 299)
Then the rule of Islam will be until 1500s. Thus, degeneration and then Doomsday is to be expected between the years 1500-1600.
Beddiuzaman Said Nursi states that the life span of the community will be up until 1506.
"The first statement, with reference to 1500, indicates the last times of those who struggle for religion in the End Times; the second statement, with reference to 1506 calls attention to the date of the victorious struggle.
(...) until this date (1506), it hints that it will, evidently and clearly, continue in a victorious manner." (Sikke-i Tasdik-i Gayb, p. 46)
Like Imam as-Suyuti, Bediuzzaman also indicated that the prevalence of Islamic moral values will continue till 1500s (Hegira) and then degeneration will commence.
In another statement of the related matter of Imam Rabbani revealed that the Mahdi will appear at the beginning of the century.
Yet his coming (of the blessed Mahdi) will take place at the beginning of the century. (Letters of Rabbani)
If the community of Muhammad does not exceed over 1500, there is only one beginning of a century is left to reach 1500 (Hegira). And this is the beginning of 1400s (Hegira). Thus, the time of the appearance of the Mahdi is the beginning of 1400s.
As our Prophet (saas) mentioned in the hadith, there will be a time of the prevalence of Islamic morality and after passing one century there will be a prevalence of tyranny and unbelief for nearly a century, then the Doomsday will be expected:
The Messenger of Allah (saas) said:
Doomsday will not take place until a century in which Allah is not worshipped has passed.
(Al-Burhan fi Alamat al-Mahdi Akhir al-Zaman, p. 92)
In the light of the hadiths of our Prophet (saas) and the statements of Islamic scholars, the life of the community of Islam will not exceed the year 1500 (Hegira style). Thus, the Mahdi should come in the beginning of 1400 (Hegira style).