Wasn't there a time when they claimed Earth was around 42,000 years old? Based on a six-day creation made up of 7,000-year "days," or something equally nutty.
Granted, it's based on the same literalist thinking that gives us a 6,000 year old planet, so you could come up with a range of values. They're all wrong, which is what you get when your unit of measure is "whatever these ancient people pulled out of their asses." But, as with so many other beliefs, the WTS found ways to differentiate themselves for no other apparent reason than wanting to be different.