Wait For Me Yesterday in Spring by Mei Hachimoku

Rescue the Past
After disturbing experiences in the city, seventeen-year-old Kanae flees back to his childhood home on a remote island. Once there, he reunites with his old friend and first love, Akari. But suddenly, Kanae finds himself propelled four days into the future. Akari's older brother is now dead and Kanae has no memory of what happened in the intervening four days. Can Kanae travel back in time to prevent the death of Akari's brother?