Test of Time is the 22nd episode of Season 3.
When Billy forgets to do his history homework, Grim gives him a universal remote control to alter time and space so he can go back in time and finish his report. Then, Billy and Mandy travel to the American Revolution, to prehistoric times, and even to the future, where they meet an aged Billy who is just finishing the report. After a struggle for the remote, Billy ends up again in the Prehistory and the remote breaks. Fast forward into the present, every person is dumb like he is and shares his characteristic big nose, including Mandy and Irwin.