And it might be one of the coolest things I've ever seen in the game.
Actually scratch that. It's not necessarily the clock itself that's interesting, it's a combination of things: the motivation for creating the clock, and just how much will happen in this universe as that clock continues to tick down towards the extinction of all known matter.
YouTube user spumwack had a noble goal in creating this clock: he wanted to make the most useless thing in Minecraft possible. He came up with a solution: a door that would only open after the universe had ended. Pretty useless.
But the video that documents the creation of the timer, and goes into the reasoning behind it, is absolutely fascinating. It's a lesson in perspective. It's actually quite beautiful and I love it.