Q: What do you call a dog that can tell time?
A: A watch-dog!
A watch is something you use to tell what time it is; a watch is typically worn on your wrist although some people use pocket watches. (A clock also shows the time but is usually on a wall or on a table.)
This joke is funny because it makes you think of a dog wearing a watch: A watch-dog. Can you imagine a dog wearing a watch, telling time?
While these dogs below may not be able to tell time, they do a great job of herding: