I have posted on here a couple of times that I am reading Alan Rogerson's excellent book entitled "Millions Now Living Will Never Die."
This book was published in 1969. Notice what Mr. Rogerson (a NON-JW) observed in his research.
Jehovah's Witnesses do not expect the literal destruction of the
planet earth at this final showdown which the Bible refers to as
the Battle of Armageddon. They also stress that no one on
earth will know exactly when it will happen: 'But of that day and
hour knoweth no man, no, not the angels of heaven, but my
father only' (Matthew 24: 36). In 1966 through the books and
magazines the Society delivered a new truth: according to their
calculations, 6,000 years from the creation of Adam ended in the
year '1975' We have already seen that the Witnesses think each
creation day was 7,000 years long and that we are still living in
the 'seventh day' when God rested; also the Witnesses believe the
last 1,000 years of this seventh day must be the promised Millennium
when God will restore perfect conditions to the earth."
The implication is that the Millennium starts in 1975 and hence
the Battle of Armageddon must occur before then!