Until the 1980s the Watchtower Society dogmatically taught that a creative day was 7,000 years long. In the early 80s one or two comments in WTS literature used the term "millenniums" instead of "7,000 years". This was used again in the 1985 Creation book. The last mention of 7,000-year 'days' was in a 1987 Watchtower article, so far as I know, and the 1988 Insight book, which is supposed to be authoritative, uses "millenniums". So the usage is today.
JWs don't really know what to make of this, since any who were JWs before the mid-80s is well aware of the older 7,000 year teaching. But because the Society has not explicitly abandoned it, and is using the non-committal "millenniums" term, some will say that the Society no longer teaches the old idea, while others will argue that because the last definite statement mentions 7,000 years, that is still officially the teaching.
I think that the Society's leaders are split over the issue, and so can't come to a consensus, and therefore don't want to alarm the rank & file. If they abandon the old idea, that would create a huge problem for older JWs who understand that JW chronology is very dependent on it. Newer JWs generally know very little of the traditional importance, and they accept the fact that life has been on the earth for at least a billion years, and so it's not an issue with them. I think that eventually, after most of the old guard has died off, the Society will explicitly accept modern dating methods.
AlanF