It appears that the only date that the Watchtower cannot change is 1914. It is at the center of their fraudulent claim that they are a 'prophet' who are able to predict world events (Like WWI).
As far as to how they came up with 1914 - it was supposed to be the beginning of the Millennium - it was biblical chronology that was the key line of 'evidence' with the pyramids merely serving to corroborate it. The pyramids were considered important but not the primary argument.
As far as Joseph Rutherford is concerned he jettisoned the whole pyramid nonsense and even said it was of the devil.