The word, "reign" is not found in the Greek text. The word is Kathizo, 2523; another form of 2516 (to sit down, to sit) to settle - continue, set, sit (down), tarry.
He doesn't stay there and reign for any period of time such as 3.5 years. This thinking has led many to not realize the Man of Sin was Titus. He was the only foreign ruler since Paul spoke of him to sit down as a god in the Temple of God. Then the temple was destroyed never to be built again.
AND now the Romans, upon the flight of the seditious into the city, and upon the burning of the holy house itself, and of all the buildings round about it, brought their ensigns to the temple and set them over against its eastern gate; and there did they offer sacrifices to them, and there did they make Titus imperator with the greatest acclamations of joy.
He doesn't stay there and reign for any period of time such as 3.5 years. This thinking has led many to not realize the Man of Sin was Titus. He was the only foreign ruler since Paul spoke of him to sit down as a god in the Temple of God. Then the temple was destroyed never to be built again.
AND now the Romans, upon the flight of the seditious into the city, and upon the burning of the holy house itself, and of all the buildings round about it, brought their ensigns to the temple and set them over against its eastern gate; and there did they offer sacrifices to them, and there did they make Titus imperator with the greatest acclamations of joy.
17 so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name.
I do not think it happen in Titus era.