Moments ago I read something fascinating regarding the WT's calculation of 6,000 years of human existence in an ex-JW scholarly book by M. James Penton called Apocalypse Delayed: The Story of Jehovah's Witnesses (1st Edition) on pages 199, 201, and in footnote 69 on page 346, which led me to something even more fascinating in the February 15, 1955 WT on pages 94 - 95 (as published in the 1999 WT Library CD-ROM).
Penton's book said in the 1943 WT book called The Truth Shall Make You Free that the WT said the 6,000 years of human existence would end in 1972, but was later shifted to 1975, then 1976, and back to 1975 again. The 1955 WT (in the "Questions from Readers") answers the question of: "What are the reasons for changing the date of Adam's creation first from 4028 B.C. to 4026 B.C. and now recently in the book "New Heavens and a New Earth" to 4025 B.C.?" The latter part of the answer says the following (I added boldface for emphasis)..
'According to Genesis 1:24-31 Adam was created during the last part of the sixth creative-day period of 7,000 years. Almost all independent chronologists assume incorrectly that, as soon as Adam was created, then began Jehovah's seventh seven-thousand-year period of the creative week. Such then figure that from Adam's creation, now thought to be the fall of 4025 B.C., why, six thousand years of God's rest day would be ending in the fall of 1976. However, from our present chronology (which is admitted imperfect) at best the fall of the year 1976 would be the end of 6,000 years of human history for mankind, 6,000 years of man's existence on the earth, not 6,000 years of Jehovah's seventh seven-thousand-year period. Why not? Because Adam lived some time after his creation in the latter part of Jehovah's sixth creative period, before the seventh period, Jehovah's sabbath, began.
Why, it must have taken Adam quite some time to name all the animals, as he was commissioned to do. Further, it appears from the New World Bible Translation that, even while Adam was naming the animals, other family kinds of living creatures were being created for Adam to designate by name. (Gen. 2:19 footnote d, NW) It was not until after Adam completed this assignment of work that his helpmate Eve was created. Since God created nothing new whatever on the seventh day, Eve must have been created on the sixth day; and this the divine record confirms in its account of the sixth day: "God proceeded to create the man in his image, in God's image he created him; male and female he created them."-Gen. 1:27, NW.
The very fact that, as part of Jehovah's secret, no one today is able to find out how much time Adam and later Eve lived during the closing days of the sixth creative period, so no one can now determine when six thousand years of Jehovah's present rest day come to an end. Obviously, whatever amount of Adam's 930 years was lived before the beginning of that seventh-day rest of Jehovah, that unknown amount would have to be added to the 1976 date.'I don't recall ever reading the WT saying that Jehovah God created more animal family kinds after the creation of Adam
, but perhaps I read such in the latter 1970s. But a number of years ago while doing independent study of the Bible I did get the idea of Genesis 2:19 possibly conveying that idea.