Natsuiro no Sunadokei

Makimura Koutarou has made up his mind: during the summer vacation, he is going to confess his love to the seemingly unapproachable Serizawa Kaho. Yet, on the night before, he has a run-in with a strange girl - an encounter which propels him into the future. There, he learns that not only has Kaho-chan become his girlfriend, but she has also died in a tragic accident. Returning to his own time, Koutarou sets out to change the future and save his love.
(Source: Anime News Network)