In the beginning God created the heavens and the earth" .
The WT/JW `s have stated in their publications that this could have taken place millions if not billions of years ago ,they don`t know .
However in their publications eg .the" Paradise lost Paradise Regained" book I think it was in there they stated that it was only when God turned his attention to populating the earth with humans , that the creative days were determined to be 7000 years long .
And not from the beginning of creation in Gen 1:1
And that was because human recorded history only goes back about 6000 years ,and the GB reckoned that it would now be a good time for Jesus Christ to begin his 1000 year reign .
And that`s how they calculated 6000 years back from then which gave them 1975 ,and projected JC reign to go for another 1000 years.
The creative days of 7000 years only applies to when God turned his attention to the earth and making it a paradise for humans .
I hope that clarifies it all somewhat .