I have been doing some reading about the Millerites and the "Great Disappointment" of 1844. I cant help but to notice some similarities between the Millerites and Russell.
The Millerites also used a prophecy of 2,520 years, but chose a different starting point. They also appear to have initially forgotten the zero year, but later corrected their mistake.
I think it's interesting that the Millerites had two prophecies that they thought pointed to 1844 (one dealing with 2,520 years and another with 2,300 years).
This kind of makes sense (if you try to relate to their world view), because the Bible says that you need two witnesses to establish something. A person can shuffle numbers in the Bible all day, but it's not likely to mean much. However, if you find two prophecies that seem to point to the same time period, then you might have something. It would make sense to ignore numerical speculations, unless there was some kind of confirmation. You wouldn't expect God to provide a lonely prophecy that would just get lost among an endless stream of idle speculations.
Russell followed this same model when he used a prophecy in Daniel and pyramidology to point to 1914. The JWs still accept the prophecy in Daniel but have renounced the pyramidology. That leaves only one witness to testify to 1914. They can't claim that the fulfillment is the proof because their expectations about 1914 failed. There current understanding came after the fact.
It seems that even if the other problems with 1914 were overcome (Jerusalem not being destroyed in 607 BCE, no mention of a second fulfillment of the prophecy, uncertainty about the number of days in seven times, and so on), there still wouldn't be a Biblical basis to accept the prophecy as anything more than idle numerical speculations.
Some might point to Jesus' prophecy about the last days, but this gives no way of predicting the date 1914. There also isn't any Biblical indication that Jesus' words and the prophecy in Daniel are talking about the same thing.
So where is the second witness?