Animal House/WMG

D-Day is a Time Lord
He decided to spend some time observing 1960s college life in the most Badass way possible, so he created a whole Badass Biker persona to fit the time period and spent four years getting into as much Wacky Fratboy Hijinx as humanly possible. He didn't need to get a degree because he already had centuries of education under his belt, so he barely showed up for any of his classes (hence, he has no GPA). By the time the parade riot came around, he decided he'd seen enough of 1960s America, so he went off to explore another corner of the universe (hence, his whereabouts are unknown at the end of the movie).