Okay, try to pay attention because this gets tricky. If you get confused, simply start over at the beginning. It's important for me to ramble this with confidence so you will be sure to believe me. I cannot say that there is nothing in the Bible, because that's not what you want to hear, but we have to put some things together in the Bible to arrive at 7000-year (not 6000-year) creative days.
How was the 1975 prophecy derived from the bible?
Rev. 20:6-7 says that the millennium will be a 1000 year reign with Christ.
Ps. 90:4 and 2 Pt.3:8 say that a day with Jehovah is 1000 years, the millennium will be Jehovah's 1000 year day of rest after the creation.
At the same time, the entire 7000 years will be Jehovah's day of rest, a seventh day. (Remember if you are confused, start again at the beginning.)
It's like a double meaning: this Sabbath day (1000 years of Christ's reign) must have been preceded by six 1000 year days since the creation of Adam and Eve at the end of the sixth day creative day. (We are now in the last "day" of the seventh day of creation. It's a 1000-year day within a day kind of thing.) Gen.1:27 The Bible says that the seventh day must end with Jehovah saying "and it was good" and that won't happen until the 1000 year reign is up, so this necessitates the double meaning where there is a week of days in the "seventh day." (Start over if you are confused.)
Since the last creative day will last 7000 years, the other creative days must have also been periods of 7000 years. Jehovah is a God of order, so of course they are all the same length (I forget the scripture for "God of order" and it adds weight here). So from the beginning of the first creative day to the creation of Adam and Eve was 42,000 years. And since "reliable Bible chronology" establishes 4026 BCE as the date of Adam's creation, we know that the first day of creation occurred 48,026 years ago. (And Armageddon and the millennium are 26 years overdue.)
Jehovah ends his sixth day of creation by fashioning Eve from Adam's rib. The seventh day of creation began in 4026 BCE and will last 7,000 years. Armageddon will occur in "the autumn of 1975, fully 6000 years into God's seventh day, his rest day." (April 1, 1968, Watchtower) Gen.2:22
The above was slightly off. We figured that when the end didn't come in 1975. So we forget that Eve must have been created some years instead of some months after Adam. Well, Adam was 130 when Seth was born. The seventh creative day must have ended before that as none of the children were born in the sixth creative day- after all, everything was still "good" at that point. So, take a few years off for Eve to have Cain and Able and for them to grow up a bit, say 20 years off the 130 total. That still gives us until 2085 for the last "day" of 1000 years to get started to be included in the creative day of 7000 years.
Was that clear? Did I throw in enough scriptures, because I can choose many that don't contradict my explanation. I know you were looking for a smoking gun, but Jehovah just doesn't work that way.