I posted this on this forum ten years ago:
Here's the real simple version of the complicated doctrine of when Jesus supposedly returned and the last days arrived/ended/started.
William Miller said that Jesus' Return would be in 1844. (See The Great Disappointment at http://en.wikipedia.org/wiki/Great_Disappointment ) People who believed what he said were known as "Millerites." It (Jesus' return) didn't happen. This return of Jesus was also known as his second advent, and believers came to be known as Adventists.
Adventist Jonas Wendell suffered in his faith at the Great Disappointment, but later carried the same type of "Millerites" talk in a sermon where Wendell had picked the new date of 1873 or 1874 for Jesus' 2nd return (after studying Bible chronology and being wrong about 1868). 18 year old C. T. Russell heard Wendell in 1870 speak of such a date. Wendell had already published The Present Truth, or Meat in Due Season highlighting such dates. So Wendell invented those terms that would haunt the WT organization and morph into New Light and food at the proper time.
Wendell died in August of 1873, probably confident that his 1873 to 1874 date was correct. When Jesus didn't appear as expected, Russell still believed it (or at least wanted to keep selling pamplets that explained it). He taught that Jesus must have returned invisibly, meaning that Jesus was ruling from Heaven starting in 1874. He taught that 40 years later, a literal application of how long a "generation" was, in 1914, Jesus would take power on the earth. That would mean an end to the Gentile Times in 1914 and an end to the last days- destruction and death and stuff.
Because World War One (coincidentally) started in 1914, Russell used that as a sign that he was correct in his complicated doctrines. He was already good at reinventing understanding rather than admitting he was wrong.
All the rest is just calculated backwards- the stuff about 607 BCE and the 2520 years and the Seven Times.
All of it was just made to fit that understanding. When things didn't work out as planned (again) the people in charge just reinvented the understandings and adjusted the math to fit whatever they wanted it to fit to sell more pamphlets/magazines/books. The end of the last days became the beginning of the last days. 1874 was forgotten. They just kept changing the end-date. They even made terrible errors and just changed the math to fit- they forgot there was no "Year Zero" and changed 606 BCE to 607 BCE.
They kept 1914 because it was so well-known to the followers and they already said "See, we were right" when World War One started. Everything about Nebuchadnezzar and the destruction/building of the temple was just made to fit the math. You can find anything if you work backwards. It's like writing a Sherlock Holmes story. Just work backwards from what a great detective would expect to find and make him look like a genius in discovering those things.
It is so unnecessary to understand the ridiculous theories of WT's present understanding when you can see what it was based on and how it changed to suit their need to get a following and sell pamphlets/magazines/books. It saddens me that I bothered to understand it at one time.