Chronos - The Forgotten Weapon

When the Straw Hat Pirates discover records of a fourth, lost ancient weapon inscribed on the Rio Poneglyph, the instructions for it's creation are given to their captain in the hope that he may escape their coming demise with it. Its name was 'Chronos', named after the god of time, the field in which it acted. With Luffy sent back in time, how will it change the future? -Harem-