The answer comes from the beatiful article "Why are you looking forward to 1975" printed in the watchtower, Aug 15, 1968:
I'd love to copy it out because not only is it hillarious but only the article that caused thousands to leave. But, alas, it's a long one. It basicly says that the start of the seventh creative day began AFTER Adam had named all the animals and Eve was created, but BEFORE they sinned. They say that Adam was created 6000 before 1975, but the day ended later.
The 7th creative day is 6000 years long + 1000 years of christs reign.
Here are the highlights:
Why Are You Looking Forward to 1975?
WHAT about all this talk concerning the year 1975? Lively discussions, some based on speculation, have burst into flame during recent months among serious students of the Bible. Their interest has been kindled by the belief that 1975 will mark the end of 6,000 years of human history since Adam’s creation. The nearness of such an important date indeed fires the imagination and presents unlimited possibilities for discussion.
2 But wait! How do we know their calculations are correct? What basis is there for saying Adam was created nearly 5,993 years ago? Does the one Book that can be implicitly trusted for its truthful historical accuracy, namely, the Inspired Word of Jehovah, the Holy Bible, give support and credence to such a conclusion?
HOW LONG SINCE THE FLOOD?
16 Already with the help supplied by the Bible we have accurately measured back from the spring of this year 1968 C.E. to the spring of 1513 B.C.E., a total of 3,480 years. With the continued faithful memory and accurate historical record of Jehovah’s Holy Word we can penetrate even deeper into the past, back to the flood of Noah’s day.
From start of Flood
To Arpachshad’s birth (Gen. 11:10) 2 years
To birth of Shelah (11:12) 35 “
To birth of Eber (11:14) 30 “
To birth of Peleg (11:26) 34 “
To birth of Reu (11:18) 30 “
To birth of Serug (11:20) 32 “
To birth of Nahor (11:22) 30 “
To birth of Terah (11:24) 29 “
To death of Terah in Haran, and
Abram’s departure to Canaan
at age of 75 (11:32; 12:4) 205 “
Total 427 years
25 Adding these 427 years to the year 1943 B.C.E. dates the beginning of the Deluge at 2370 B.C.E., 4,337 years ago.
6,000 YEARS FROM ADAM’S CREATION
26 In a similar manner it is only necessary to add up the following years involving ten pre-Flood generations to get the date of Adam’s creation, namely:
From Adam’s creation
To birth of Seth (Gen. 5:3) 130 years
To birth of Enosh (5:6) 105 “
To birth of Kenan (5:9) 90 “
To birth of Mahalalel (5:12) 70 “
To birth of Jared (5:15) 65 “
To birth of Enoch (5:18) 162 “
To birth of Methuselah (5:21) 65 “
To birth of Lamech (5:25) 187 “
To birth of Noah (5:28, 29) 182 “
To beginning of Flood (7:6) 600 “
Total 1,656 years
27 Adding this figure 1,656 to 2,370 gives 4026 B.C.E., the Gregorian calendar year in which Adam was created. Since man naturally began to count time with his own beginning, and since man’s most ancient calendars started each year in the autumn, it is reasonable to assume that the first man Adam was created in the fall of the year.
That means, in the fall of the year 1975, a little over seven years from now (and not in 1997 as would be the case if Ussher’s figures were correct), it will be 6,000 years since the creation of Adam, the father of all mankind!
. Our chronology, however, which is reasonably accurate (but admittedly not infallible), at the best only points to the autumn of 1975 as the end of 6,000 years of man’s existence on earth. It does not necessarily mean that 1975 marks the end of the first 6,000 years of Jehovah’s seventh creative “day.” Why not? Because after his creation Adam lived some time during the “sixth day,” which unknown amount of time would need to be subtracted from Adam’s 930 years, to determine when the sixth seven-thousand-year period or “day” ended, and how long Adam lived into the “seventh day.” And yet the end of that sixth creative “day” could end within the same Gregorian calendar year of Adam’s creation. It may involve only a difference of weeks or months, not years.
33 This time between Adam’s creation and the beginning of the seventh day, the day of rest, let it be noted, need not have been a long time. It could have been a rather short one. The naming of the animals by Adam, and his discovery that there was no complement for himself, required no great length of time. The animals were in subjection to Adam; they were peaceful; they came under God’s leading; they were not needing to be chased down and caught. It took Noah only seven days to get the same kinds of animals, male and female, into the Ark. (Gen. 7:1-4) Eve’s creation was quickly accomplished, ‘while Adam was sleeping.’ (Gen. 2:21) So the lapse of time between Adam’s creation and the end of the sixth creative day, though unknown, was a comparatively short period of time. The pronouncement at the end of the sixth day, “God saw everything he had made and, look! it was very good,” proves that the beginning of the great seventh day of the creative week did not wait until after Adam and Eve sinned and were expelled from the Garden of Eden.