Some say Shaykh ul Hind Rh and some say Maulana Ashraf Ali Thanvi Rh. However, I think Maulana Qasim Nanotvi Rh and his two other companions rh and there is a reason behind it. He started the Deoband movement and it is his students who initiated Tehreek e Khilafat, Tabligh Movement, Massive ilmi, islahi, jihadi movements etc. So by starting the Deobandi movement, he deserves the title of Mujaddid e Azam (this is out of love for this saint and nothing shari in it and my personal opinion).
If one takes your argument of hijri calender into consideration and if we accept Maulana Ashraf Ali Thanvi Rh as the mujaddid then as he died in 1943 which means that Imam Mahdi will come around 2043.
If we accept Shaykh ul Hind Rh as the mujaddid then he died in 1920 and according to the objection you made, Imam Mahdi should be coming in 2020 or around that year.
If Imam Mahdi is very far away then I would declare Mullah Mujahid Umar Rh as the mujaddid of current century.
Don't focus on the time of his arrival. Just get prepared. Even if he is 1000 years away, history is telling us again and again that when a nation, including Muslims, becomes active, she is the winner and inactivity leads to disaster and failure.
Now translate activeness and in-activeness as you like.