TD - I think the 120 years idea comes from Genesis 6:3
I know. That's not in question. The question is how to draw a legitimate parallel between that 120 years and a modern 120 year ministry starting in 1914?
Noah did not have 120 years warning and he did not spend 120 years preaching a message of impending judgment. The Witnesses own publications (e.g. Insight) estimate that Noah only had about 50 years advance notice.
(I'm saying this wearing my fundamentalist 'hat' of course. 6:3 was probably inserted by the redactor who knitted together the J and P accounts.)