Ray Franz' book crisis of conscience , has an interesting chapter "predictions and presumption", which deals with the history of the 2520 years thing.
It was John Aquila Brown, 50 years before Russell, who started thinking that way.
NHBarbour, a 7th day adventist, started publishing a magazine, pointing to 1914, and Russell, seeing it, adopted the same idea.
Without getting too bogged down in studying, which I dont particulaly like ( not got the time),
its interesting to wiki "Second Great Awakening"
It seems that in the USA, there were a lot of people looking for the 2nd coming of Christ........
and ended up searching for signs and secret codes. DAN BROWN would have been in his element.