According to JW Bible chronology, Adam was created 6000 years before 1975 (4026 BCE). JW figured that the Millennium would begin soon after 1975 because according to JW doctrine, each Creative Day is 7000 year in length. And the 1000 year long Millennium adds up perfectly with the 6000 years ending in 1975.
And based on the belief that humanity is living during God’s Sabbath: (“And by the seventh day, God had completed the work that he had been doing, and he began to rest on the seventh day from all his work that he had been doing. 3 And God went on to bless the seventh day and to declare it sacred, for on it God has been resting from all the work that he has created, all that he purposed to make. COMPARE “For this reason the Jews were persecuting Jesus, because he was doing these things during the Sabbath. 17 But he answered them: “My Father has kept working until now, and I keep working.”’—Jesus was alluding to the ongoing Creative Sabbatical Day mentioned in Genesis.
Since the ongoing Creative Sabbath Day should end by 7000 years based on JW belief that it is 7000 years long, it was JW belief that 1000 years Millennium ( starting on 1975 ) after the 6000 year old Creative Sabbath would fit perfectly in time to fulfill God’s purpose for humanity: 6000+1000=7000= end of 7th Creative day. And at the end of day, God would declare the 7th Creative Day: GOOD.