Karesen Joshi Kousei to Tokikake Oji-san

Summer 1998 - me (40) and her (17) begin our youth.
A 40-year-old businessman slips back in time to his youth! On the outside he is a young fit high school boy but inside, he is a model uncle. A nostalgic love story about a "time-traveling uncle" and a high school girl who loves his uncle-like appearance.
(Source: Pixiv Comic, translated)