They do mention that the earth is billions of years old now. Here's a quick one I found at jw.org faq:
Age of the earth. Some creationists teach that the earth is just a few thousand years old. However, according to the Bible, the earth and the universe existed before the six days of creation. (Genesis 1:1) For that reason, Jehovah’s Witnesses have no objection to credible scientific research that indicates the earth may be billions of years old.
But Master_Bob is right, they do believe the Earth was there BEFORE the creative days began, which of course makes no sense. Here's a quote from the Insight book under "Creation":
Genesis 1:1, 2 relates to a time before the six “days” outlined above. When these “days” commenced, the sun, moon, and stars were already in existence, their creation being referred to at Genesis 1:1. However, prior to these six “days” of creative activity “the earth proved to be formless and waste and there was darkness upon the surface of the watery deep.” (Ge 1:2) Apparently, a swaddling band of cloud layers still enveloped the earth, preventing light from reaching its surface.