So this is probably pretty useless, but I found it interesting.
In the newest Minecraft Snapshot (13w16a) you can "leash" mobs and tie them to a fence post. So, I had a cow hanging from a fence post (I know, I'm sick), and noticed when I hit it, the wolf I had tamed made it's way up the hill/trees to attack the cow. He would jump and miss every time and take the same path back up, resulting in semi-consistent timing. I say "semi-consistent" because occasionally the wolf will turn or hesitate instead of immediately following the path.
Like I said, probably useless, but an interesting concept for a clock/timer.