The year 1914 was first calculated by measuring the inches inside of a pyramid in Egypt. It was from an inner chamber to somewhere else. The original calculation showed the year 1874 as being the end. When this failed they had to change the length of an inch to "pyramid inches" to come to 1914.
After chuckie russel died the corporation had to distance itself from the quackery of pyramidology. They adopted the 2520 days equals a year quackery to support their 1914 doctrine.
as for calculating leap years and prophetic years verses calander years mumbo jumbo it really doesn't matter because Jerusalem was destroyed in 587 not 607 bce.
To compound the problem is the fact that the generation of 1914 was not supposed to pass away is that they are all pretty much dead now. So now they have to make up a bullshit definition of generation to try to keep 1914 relevant to the religion.
as far as I am concerned the leaders of this religion have been bullshitting the members for the last 150 years and will do so for another 150 years as long as the gullible buy into it. It really doesn't matter to me anymore as long as I can keep my family intact and save enough money for my old age.