The first independent biographer of Mirza Ghulam Ahmad, H.A. Walter, writes:
"was born in the village of Qadian, Gurdaspur District, Panjab, on the eighteenth of June, 1839. (Footnote: Recent references to the date of Ahmad's birth place it vaguely 'some time in 1836 or 1837' (Review of Religions, XV, p. 26), but the date given here is the one generally accepted by his biographers.), the year marked by the death of Ranjit Singh, the great Sikh ruler and warrior."
Why was the date being pushed back? Mirza repeatedly prophesied that God had told him: 'your age will be eighty years - a few years less or few more, and you will live to see distant progeny..'. Perhaps it was an effort by his followers to retroactively fulfill the prophecy.
A proof of this post-facto distortion can be seen here:
This change happened right after the death of Mirza Ghulam Ahmad, as evidenced from the Review of Religions published right after his death in June-July 1908. While reprinting the obituaries of Mirza, the Review added this note:
"We do not intend to criticize these remarks, but we might be allowed to contradict in the beginning the estimate of the adherents of the movement and the probable date of the birth of the founder which may more correctly be placed at 1836 or 1837." (Review of Religions, 1908, page 235)
As Walter has researched, Mirza was born in 1839, and died aged 69. Here are the references from his books:
When Mirza's date of birth was revised by his followers, the only citations from Mirza's writings that were used were the ones where he would state his age when throwing a death challenge to an opponent and tried to show that he was much older than the opponent and thus more likely to die. For example, 'I am approximately 60' or 'I am approximately 70’. However, wherever Mirza Ghulam Ahmad was specific, and whenever that specific information was published in his lifetime, it was ALWAYS 1839 or 1840.