Read this somewhere before 1995. Title may have had "time" in it somewhere. Was about a kid who time travels to a dystopian future. I remember very little, but I recall that he traveled there through a cave. In the end, he had a laser gun he was going to use as proof. He had to use the gun to cut his way out of the cave, and lost the gun. Last line was something like "I knew how I would tell everyone; I would write a book."