Quoted from ANN:
Japanese moviegoers who see this year’s One Piece Film Strong World will receive the “0th volume” of Eichiro Oda’s One Piece pirate manga. Oda launched his manga about the young pirate captain Monkey D. Luffy in Shueisha’s Weekly Shonen Jump magazine 12 years ago in 1997, but the manga in the upcoming “One Piece Kan-Zero” bonus gift will depict the world of One Piece of 20 years ago. In the real world, Oda was a junior high school student 20 years ago when he conceived of the earliest concepts of the pirate manga that would eventually become One Piece.
Oda-sensei is awesome.! ^_^