First off the 606 bce date was arrived in error because of the zero year between 1 bce and 1 ce. Russell erred in acounting for this zero year and so it actually came down to 1915 ce with the 2520 day for a year from 606 bce. This was left unchecked until 1943 when it was adjusted in the book "The Truth Shall Make You Free". 606 or 607 bce, it doesnt matter is still not an accurate date established by evidence but is derived so that it would fit with the 1914 chronology. They cant abandon the date 1914 because of the phrophecies they claim that followed shortly. Specifically the pivotal 1918 date in which they say Jesus came to earth to inspect all of his followers and then chose the watchtower society as his choice. To abandon 1914 for 1935 would then nullify Jesus having chosen them to take care of his earthly belongings in 1918, when they supposedly recieved a spiritual cleansing and revival. I would speculate this is the largest reason they are hesitant to shed the 1914 date but the way they have altered the meaning of the term "generation", they have bought a little breathing room for the time being. Its easier to treat the symptons then cure the problem, so its easier to adjust then abandon a position that would threaten their authority in the eyes of their followers.
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