Hello, Davidiam, and welcome to the forum!
I am the one who made the assertion that the 360-day "prophetic year" was a fabrication of the Society. Others have pointed out how this figure was derived from the language used in the book of Revelation. They have also referred to others who said the same thing back in the nineteenth century. I appreciate their research.
My argument that this is not scriptural arises from different factors. The equating of "a time, times, and half a time (3½ years)" with 1,260 days and 42 months is made only, wholly, and solely in the book of Revelation. You will not see or read this anywhere else. Furthermore, John is specifically told how long this period will be. No other Bible writer uses it. So calling it a rule for prophetic measure in the entire Bible has no support.
John lived during a time when the Jews used two different calendars. The first was their own lunar-solar calendar which had been in use for centuries. I won't go into all the details of how it worked in this space except to say that it certainly was not 360 days long. The second calendar John would have been familiar with was the Julian calendar that was the standard throughout the Roman Empire in which he lived. It had been introduced by Julius Caesar in 45 BC after his consultation with various astronomers and mathematicians. It was 365¼ days long, with a leap year observed every four years. By the time John wrote the Revelation, this calendar had been in use for a century or so and he would have been thoroughly familiar with its workings.
Now, looking at the prophecy in Daniel chapter 4, we see seven "times" mentioned. It is very important to note that the word used in Aramaic has been translated as "times". These times are not called years, months, seasons or anything else but times. And unlike John, Daniel is not given any other substitute or equivalent expressions. Daniel is not told that his seven times are 84 months or 2,520 days. Neither of those numbers appears anywhere in the book of Daniel. Since Daniel was inspired by holy spirit just as John was, his omission of other expressions is significant. The linking of Daniel's "times" with John's "times" cannot be established in light of how the expression is used in the two books, particularly when you keep in mind that Daniel wrote centuries before John did. All we do know is that Daniel spoke of "seven times" and John of 'three-and-a-half times'.
I believe the Bible to be the inspired Word of God, but I think it is essential not to go "beyond the things that are written". The calculation of the length of the seven times by William Miller and others (for Miller wasn't the only one who said they were seven "prophetic" years) is based on nothing more than their opinion. Furthermore, Miller, Charles Taze Russell, and others took Daniel 4 to have a double application, saying the tree in Nebuchadnezzar's dream somehow represents God's Kingdom. But a careful reading of Daniel 4 does not lead to that conclusion. Instead, Daniel tells the king that the dream and its fulfillment are meant for Nebuchadnezzar alone. Nebuchadnezzar believed the same thing. Daniel repeats that finding to Belshazzar. So linking the seven times in Daniel to the "appointed times of the nations" in Luke 21:24 has no scriptural basis. --Daniel 4:20-22, 34-37; 5:18-22