Why do you call it a crazy booklet of speculations?
They used scriptures that they thought were prophetic that related to their times. They came up with the 2520 years of Daniel 4 as it was yet ahead of them. They were wise for their times and it was not nonsense, simply because without that, how is it explained that Jesus never became king when it was prophesied to David that his offspring would rule Jehovah's throne forever?