Q: What do you call a dog that can tell time?
A: A watch-dog!

Explanation: A watchdog is a dog that guards your house; they are also known as guard dogs. (Watchdog can also refer to a group or agency that makes sure that other groups are doing good work.)

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: