I seem to remember from my early years that the creative days were all thought to be 7000 years long and hence why 1975 was such an important date.
The Watchtower taught that Adam was created in 4026 BCE. Add 1975 years (no year zero) and that adds up to 6000 years.
As there was no definite date for the creation of Eve there was a supposition that it may have been 30 or 40 years. After that God rested and the 7 th day began.
The 1000 reign obviously didn’t appear in 1975. Add on 40 years and et voila – 2015!
I wouldn’t hold your breath folks!
HOTB