The Time Machine belongs to Mermaid Man and Barnacle Boy. It just appears in the episode "Back to the Past." Patrick, who is quite stupid, puts a quarter inside a coin slot and claims he lost his quarter, which sends SpongeBob, Mermaid Man, Barnacle Boy, and Patrick himself, into the past. It is seen again, where SpongeBob and Patrick use it to go into the past a second time. At the end of the episode, it is seen that a lot of the time machines go into the past. According to Barnacle Boy, going through time is so dangerous, he and Mermaid Man choose to leave it alone.
The Time Machine is a large blue machine with a large, yellow sign reading "Time Travel" in large pink letters. Out of the top on both sides there are red coils with lightbulbs at the end. The words "Date" and "Location" are side-by-side at the top of the machine. Many multi-colored buttons (that do nothing) are seen at the lower end of the time machine, with a small radiator fan above.