When I discussed the year 1975 with Jehovah’s Witnesses, I heard another explanation: that 6,000 years should be counted from sin of Adam and Eve, not from their creation. Since we do not know when they committed their sin, so we cannot determine the end of 6,000 years and the beginning of the millennium. I could not find such interpretation in the publications, but we cannot rule out that such an explanation existed or that it was verbally transmitted among publishers (in early days of the Watchtower Society the return of Christ in 1874 was calculated by adding 6,000 years to a date of Adam’s sin in a year 4126 before Christ, not to a date of his creation in 4128; see Thy Kingdom Come 1919, pp. 127-128).
Let us check if The Watchtower Society determined the moment of that sin. Yes, it determined it, that it happened two years after man’s creation. Also in other publications it pointed to the same year in which it placed creation (4025 and 4026 B.C.E.). The following are older and newer statements from The Watchtower Society’s publications:
Man’s creation was placed in 4128 and sin’s entrance in 4126 B.C.E. (The Watchtower 15 August 1974, p. 507; see also God’s Kingdom of a Thousand Years Has Approached 1973, pp. 207-208);
So, then, whereas Usher dates A. D. 1 as the year 4005 from the creation of Adam, it really was, as we have shown, the year 4129, according to the Bible record... (The Time Is at Hand 1917, p. 53);
Fall 4127 B.C. Fall of Adam (The Finished Mystery 1917, p. 60);
It will be remembered that the reckoning of chronology began with the creation of Adam, and that some time was spent by Adam and Eve before sin entered. Just how long we are not informed, but two years would not be an improbable estimate. (Thy Kingdom Come 1919, p. 127);
There is some Scriptural evidence to indicate that approximately two years of time elapsed during which they enjoyed the blessedness of their home and the beauties of their surroundings and uninterrupted sweet fellowship with each other. Then began the sorrows of man, and since then sorrows have ever been present with the human race. (Creation 1927, p. 60);
Man's existence covers nearly 6,000 years (4025 B.C. to over 1,950 years A. D.) as shown by Bible chronology. (“Make Sure of All Things” 1953, p. 325; compare pp. 296, 344);
Edenic Judgment Day (c.4025 B.C.) Brought Condemnation to Adam and Eve and Disabilities to Their Offspring. (“Make Sure of All Things” 1953, p. 219; compare p. 383).
About the year 4026 B.C.E., Jehovah God had said to the Serpent that had brought about man’s downfall in the Garden of Eden... (The Watchtower December 15, 1970, p. 751). See also The Watchtower 1 November 1972, p. 648).
Also the Bible dictionary of Jehovah’s Witnesses pointed to the year 4026 B.C.E. as the year of sin and announcement of punishment:
Jehovah God, at Genesis 3:15, stated his purpose prophetically in the garden of Eden in the presence of Adam and Eve and the "serpent," about 4026 B.C.E. (Aid to Bible Understanding 1971, p. 387).
This passage was changed a bit by The Watchtower Society in the newer dictionary, removing the inconvenient date:
Jehovah God, at Genesis 3:15, stated his purpose prophetically in the garden of Eden in the presence of Adam, Eve, and the “serpent.” (Insight on the Scriptures 1988 vol. 1, p. 521).
As we see from the above, although The Watchtower Society moved the beginning of 6,000 calculation two years later, it gave them only year 1977, which passed long ago without fulfillment of any prophecy.