[The 1986 book, Worldwide Security Under the “Prince of Peace” starting on p. 182] was the last time the Watchtower Society even alluded to the creation day being 7000 years long.
There is one (slightly) later reference to the 7000 year Creative Day that I am aware of, from a 1987 Watchtower magazine.
(W 1-1-87, p30)
"Second, a study of the fulfillment of Bible prophecy and of our location in the stream of time strongly indicate that each of the creative days (Genesis, chapter 1) is 7,000 years long. It is understood that Christ’s reign of a thousand years will bring to a close God’s 7,000-year ‘rest day,’ the last ‘day’ of the creative week."
This teaching has never been denied outright, but; is not considered to be worth promoting anymore.
Unfortunately, the Society isn't always clear when a teaching has been changed or dropped. The 7000 year Day has been silently abandoned. The latest teaching doesn't put a time on the Creative Days.
(Was Life Created, Brochure 2010, p24)
"How long, then, were the creative days? The Bible does not say; however, the wording of Genesis chapters 1 and 2 indicates that considerable lengths of time were involved.
The Creative Days are no longer viewed as being 7000 years long, but as an unspecified 'considerable length of time.'