"Maybe it's some kind of transport pod!"
"I don't see a door or license plate anywhere." --Calvin and Hobbes in Mesozoic Era, talking about a dinosaur eggCalvin once made a time machine, which he and Hobbes used to travel back in time to the Mesozoic Era. Like many of Calvin's inventions, it is made out of a cardboard box. Journeys in the time machine have a tendency to fail fantastically.
The time machine first appeared in the strip for August 31, 1987, when Calvin and Hobbes originally intended to go to the Future. It accidentally ended up in the Mesozoic, or more specifically, the Jurassic, but they did not realize it. They found a huge dinosaur egg, which they assumed was a space pod. They realized their error when they were chased to their Time Machine by an Apatosaurus. The error occurred because they were facing the wrong way.
Later, Calvin and Hobbes went back in time again, this time to take pictures of the dinosaurs there and sell them later. This time around, Calvin had fixed the machine so the direction in which they time-travel was no longer determined by the way they faced. Their adventure was a success, gaining pictures of Diplodocus and Stegosaurus, until they were chased back to the time machine by a hungry Allosaurus. They only just escaped and then had the pictures copied out, only to have said pictures discredited (by Dad) to be frauds.
This is continued in The "Dimension" Saga, where Calvin wants to throw it out because his father thought it wasn't real. Hobbes eventually gets Calvin into a fight with Calvin and smash into the time machine, upside-down. Calvin apparently accidentally pushes a button, causing the time machine to take them to the Sonic universe. There are two claims of explanation for this; (one) because the box was turned over, programming it to go to a different dimension instead of going back or forward in time, and (two) because somebody programmed it to.
The time machine was used a third time when Calvin and Hobbes went forward in time from 6:30 to 8:30 to get a report (a one page story) from then. Upon arriving, it was discovered that the 8:30 Calvin hadn't done the report ether. So the 6 and 8:30 Calvins went to 7:30 to make that Calvin do the report. After a brief argument, the 6:30 and 8:30 Calvins went back to 8:30, where they discovered the 6:30 and 8:30 Hobbes had done the report while the Calvins were gone. So the 6:30 Calvin and Hobbes went back to 6:30. It was discovered the next day that the story was a mockery of the last day's events (that got an A+ and a good note on the "tiger narration").