Asatte no Houkou.

Few situations are as complicated as navigating a relationship with an ex-boyfriend's younger sister or an older brother's former girlfriend. Karada Iokawa and Shoko Nogami are forced to learn this the hard way when a magical wishing stone unexpectedly grants Karada's desire to be older by siphoning years away from Shoko. Suddenly, Shoko—a woman who has already graduated and studied abroad—is reduced to Karada's age, preparing to enter junior high school. At the same time, Karada is thrust into adulthood and must abruptly confront the true realities of growing up. As if this mystical predicament was not awkward enough, they must also figure out how to manage their existing romantic interests.