Hey, I'm gonna have to check out Mel's time line, but I'll just throw this into the mixnow. Since it comes from the story and isn't even mentioned in the film, anyone is free to ignore it, of course, but one thing Annie has them talk about in the story is the sinking of the submarine Thresher, which is said to have happened the month before they went up the mountain. Leslie did some research and found out that the Thresher sank in April 1963, so Annie is clearly sending them up the mountain in May of that year (and certainly intentionally in the story makes their last trip into the mountains exactly 20 years later, May 1983). Maybe 1963 was a low-snow year in the Big Horns or the Wind Rivers?