When The Phantom Tollbooth begins, Milo takes a lot of things for granted, particularly time. He doesn't see the importance of time and is always either wasting it or rushing through it. He's so bored, he can't seem to make the most of his moments. But during his journey through the Lands Beyond, he meets people who have very different attitudes toward time. Some of them waste it, some of them take advantage of it, and watching them do these things helps Milo realize he really should value his time. With the help of Tock, Milo learns that instead of whiling away the hours complaining of boredom, he can live life to the fullest by going on adventures of the mind.
Despite the fact that being a watchdog is his life's work, Tock is not actually that great at keeping track of time.
The selection of a watchdog as Milo's first and most loyal companion on his quest emphasizes the importance of time as a concept in The Phantom Tollbooth.