Jehovah's Witnesses - What is the reasoning that led to 1914 being considered a significant year?

Looks like there are a few questions here about Jehovah's Witnesses viewing 1914 as a significant year in the history of mankind, but not much about what their reasoning has been for that. The other question that is claimed as duplicate does not address the question. What is the **biblical/historical/mathematical/etc reasoning** that led Jehovah's Witnesses to believe in the significance of 1914?