I Hit The Button For A Million Years And Before I Knew It, I Was The Strongest.

Allen is a boy who is despised as a failed swordsman. One day, desperate for a duel with an elite swordsman who bets on leaving Kenjutsu Academy, a mysterious hermit asks him if he will press the "100 million year button." Can the mysterious object really help him train for 100 million years? (Source: Kadokawa, translated)