It was absolutely critical to the Futurist interpretation that the day for a year principle be somehow disproved, and for this reason it was made the focus of attacks:
'The futurists believed that none of the events predicted in Revelation (following the first three introductory chapters) had yet occurred and that they would not occur until the end of this dispensation.
Associated with this rejection of the historicists' harmonizing of Daniel and Revelation was the futurists' attack upon the year-day theory, so vital to the dating of the 1,260 years to 1798. At the first Powerscourt conference the announced topic for Wednesday was 'proof if 1260 days' means days or years.
The futurist position did not originate with the Plymouth Brethren. Sixteenth-century Roman Catholic commentators had countered Protestant attacks upon the papacy as the Antichrist by insisting that none of the events relating to Antichrist had yet occurred...
As has been true so frequently in the history of religious controversy, futurism did not become a real threat to the historists and an attractive alternative prophetic position until accepted by believers.'
This was a radical departure from the position held by expositors for centuries before.
Of course, the mere fact that the day for a year principle had been held to by expositors throughout the centuries does not make it right - it is proved correct by an appeal to Scripture - but what must be understood is that for such a well established exposition to be displaced took a considerable effort, and a massive attack not only on the day for a year principle itself, but on the entire method of interpretation on which it was built, the Continuous Historic (or 'Historicist'), method of interpretation.
It was this which was the real target, the Historicist method of interpretation. This method had proved its worth throughout the centuries, and had been well established as the dominant method of interpretation - and with good reason, for it had proved itself correct time and time again.
Ernest R. Standeen, ‘The Roots of Fundamentalism, British and American Millenarianism 1800-1930’, pages 36-38.