The 3.5 Years \ 42 Months \ 1260 Days began in 538 A.D., with the implementation of the Justinian Code. This code was created in AD 534, however could not be officially enacted\enforced until the siege of Rome was lifted in AD 538.
This law placed the pope at the head of all Christian groups, and ordered them to submit to his authority. It also gave him civil power of life and death over “heretics”. The beginning of a dark time period indeed.
Using the day for a year principle, 1260 years from 538 A.D. places the ending of this time period in the year 1798.
In 1798, the papacy received the apparent mortal wound mentioned in Revelation, which will be healed towards the end as the entire world follows the beast (papacy) and that antichrist system’s traditions:
And I saw one of his heads as it were wounded to death; and his deadly wound was healed: and all the world wondered after the beast.
Revelation 13:3