On a related note, did you know that the 1975 date for the end of 6000 years since creation was actually the Society's fourth swing at the calculation?
1872
“Six Thousand years from the creation of Adam were complete in A.D. 1872, and hence that since A.D. 1872 , we chronologically entered upon the seventh thousand or the Millenium.” (Studies in the Scriptures, Vol 2, 1906)
1873
“The Bible chronology herein presented shows that the six great 1000 year days beginning with Adam are ended, and that the great 7th Day, the 1000 years of Christ's Reign, began in 1873”. (The Time is at Hand, 1916, pii forward)
1972
6000 years of human history will end in 1972. (The Truth Shall Make You Free, 1943, p152 – summary, not a quote)
1975
“only seven more years from the autumn of 1968 to complete 6,000 full years of human history. That seven-year period will evidently finish in the autumn of the year 1975” (AW October 8 1968 pp. 14-15)
So, it's not too surprising that that still haven't hammered out the details. What's an extra hundred years or so?