The number 2,520 years is a number arrived at by using the 'special math-magic formula' of 7x360=2,520.
The number 7 comes from the book of Daniel chapter 4 regarding Nebuchadnezzar's 7 years of insanity. (As we know, Nebuchadnezzar was king of Babylon and Babylon came to be the WTBTS's symbol for all false religion. Revelation chapter 14 speaks of "Babylon the Great".)
The number 360 comes from the number of days in a 'lunar' or Jewish year. (Of course we don't live by the lunar calendar and the 'marked' year 1914 C.E. is a Gregorian calendar date.)
Here's where the 'special math-magic formula' comes into play. 7 (years) x 360 (days) equals 2,520. BUT--- since we live in the Gregorian calendar world, and this 'prophecy' is fulfilled according to the Gregorian calendar date of 1914 C.E.; 7 (years) x 365.25 (days) equals 2,556.75. A difference of almost 37 years. (Of course, by using 'math-magic' those 37 years become 'invisible' just like Christ's return was 'invisible'.)
Another part of this 'math magic formula' is assuming that 2 other Bible Scriptures, in Numbers and Ezekiel, that speak of a 'day for a year' is part of this mysterious formula. (WTBTS say that the 'day for a year' formula is a Biblical rule when meting out punishment.)
One would also have to assume also that Jehovah used the kingdom of Babylon (world empire of false religion) to forshadow/picture God's Kingdom. That somehow Nebuchadnezzar's 7 years of insanity forshadowed/pictured a period of inactivity for God's kingdom. With Jehovah (and a bit of 'math-magic') all things are possible.
Is this the kind of stuff Dan Brown used to write the Di Vinci Code?
Eyeuse2badub