What happens next is that Calvin and his future self decide to go back in time halfway and force THAT Calvin to do it. Of course, the Calvin of that time doesn't want to do either; the three of them all end up fighting each other, while Hobbes ends up writing the story that Calvin was supposed to do for homework. Calvin then turns it in without reading it... he gets an A, but he's mad at Hobbes anyway because, when he gets it back, Calvin realizes that the story (which was about Calvin trying to use his time machine to get out of writing a story for homework) made him look like an idiot.