Jan, check this out it may help you to understand and a refresher course for those who might have forgotten.
https://jwfacts.com/watchtower/1975.php
1975 Watchtower Quotes
The idea that 1975 would culminate in the end of the world was introduced in 1966.
"The published timetable resulting from this independent study gives the date of man's creation as 4026 B.C.E. According to this trustworthy Bible chronology six thousand years from man's creation will end in 1975, and the seventh period of a thousand years of human history will begin in the fall of 1975 C.E. So six thousand years of man's existence on earth will soon be up, yes, within this generation. So in not many years within our own generation we are reaching what Jehovah God could view as the seventh day of man's existence. How appropriate it would be for Jehovah God to make of this coming seventh period of a thousand years a sabbath period of rest and release, a great Jubilee sabbath for the proclaiming of liberty throughout the earth to all its inhabitants! This would be most timely for mankind. It would also be most fitting on God's part, for, remember, mankind has yet ahead of it what the last book of the Holy Bible speaks of as the reign of Jesus Christ over earth for a thousand years, the millennial reign of Christ. It would not be by mere chance or accident but would be according to the loving purpose of Jehovah God for the reign of Jesus Christ, the 'Lord of the Sabbath,' to run parallel with the seventh millennium of man's existence." Life Everlasting in Freedom of the Sons of God 1966 pp.26-30