I doubt they would ever explicitly state a date, but I'm sure they have been tempted to at least "hint" at one.
So far they've resisted the urge.
However, they have explicitly stated, and more than once, that the GT must come before the current crop of GBers die off. The only one under age 60 is Sanderson, but his obesity will shorten his life unless he starts taking significant action soon. The other guys are in their 60's & 70's with Herd in his mid-80's. So the implication is that the end "must come" within 15-20 years....so 2034-2039.
Hmmm...come to think of it, maybe they are "hinting" at a date after all.