Order a Gate Key for your Crono costume, attend a costume party with your friends. Normal enough. What isn't is the darn thing WORKING. Hello ponies, don't mind me. I'm just going to meddle with the time-stream...
Crossover/continuation with Chrono Trigger & Chrono Cross: Rarity is sent on a journey through dimensions in the hopes of finding the way back to her own.
Starlight won, or so she thought. Changing time without waiting for Twilight's return, she thought she succeeded. Upon leaving the past, she comes to a place she's never seen before. Fortunately, more than one pony in Equestria has fiddled with time.