The difference in the date of birth of the twelfth Imam is of very little vaniance comparing to that of the other Imams. Some narrations say the date of Mehdiâ€™s birth is at parity with the number of letters in the word NOOR that is light according to the â€˜ABJADâ€™ calculations. The more acceptable statement is that of MR. Fazl Bin Shazan, coeval with Imam Hasan Askari, who indicates 255.
The 12th Imam birth was surrounded by conditions not usual or normal. His absence was more than his appearance. His father, Imam Askari allowed only a few chosen associates of his own to see his son and become blest by his appearance. His short absence commenced as soon as he became Imam. In his absence he appointed deputies who were his agents. This was immediately after the death of his father Imam Hasan Askari.
Adapted from: “A Reply To Belief Of Mahdism In Shia Imamate” by: “Ayatollah Lutfollah Saafi Golpayegani”