In Norton Juster's The Phantom Tollbooth, Tock enjoys his job as a watchdog, and he is perfectly suited for it, too, for he is a dog who has a clock as a body. Tock often patrols the Doldrums, trying to get people to stop wasting time and to use their time wisely.
Tock is the first creature Milo meets when he enters into the Lands Beyond, and Tock becomes his faithful companion on his journey to rescue the princesses Rhyme and Reason. Milo is actually a little scared of Tock at first because Tock is rather gruff. Tock explains that watchdogs are supposed to be like this, but he mellows quickly and becomes Milo's best friend.
Tock actually teaches Milo a lot. Milo has the bad habit of wasting time, and he doesn't appreciate the value of time at all. Rather, he tends to perform many useless tasks. Tock teaches Milo that time is valuable even though it seems to be plentiful. He also teaches Milo to make better choices in using the time that he has and about how time seems to move in different ways in different situations.
Indeed, Tock proves to be a devoted and extremely helpful watchdog, for after Milo and Tock rescue the princesses, Milo, Rhyme, and Reason ride on Tock's back as the watchdog flies through the air, proving that time really does fly.