Happî endo no monogatari (1991)

High school student Mikiko travels back in time to the future using a time machine built by Monjiro, the physics club advisor. What awaited her there was the reality that her best friends, Yoko and Tatsuro, were already married and were on the verge of breaking up. Mikiko returns to the modern world with the help of future Monjiro to mend their relationship.