TD is correct.
Now 1874 was the original date, based off of they caculated 6000 years from Adam and that 1874/75 would start the 7,000 year and make sense to be the 1,000 year reign. When nothing happened, Russell went off of (this generation) and since a biblical generation is 40 years added that and you get to 1914. THEN he read in the bible about the "two testaments" or 2 witnesses in egypt, (some scripture in PR or IS) and took it to mean the Pyramid of Gizeh, since it was so perfectly made and appeared to be the center of the earth if using a certain map. Then measurements seemed to fall in line with dates, hence he used the pyramid as further evidence of 1914. Probably wasn't till the 20's + that all of it changed and they found a way to toy w/ the scriptures to make it look like gentile times prophecy and no way related to how it was originally predicted.
THEN in the 50's or 60's, they found that they were off by 100 years, and thats where the 1975 prediction came in. I'm surprised after that didn't happene, they didn't add the 40 years as well, making it 2014, but if that did that, it would destroy the 1914 date.