I don't read One Piece, but I thought that time DOES pass, it's just the characters don't age.

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Before I cut it, just want to check - Is Ozy And Millie actually an example? It seems to use a different convention; the same convention that's the reason that Bart Simpson is eternally ten and that Charlie Brown (eventually) reached age 8 and then stayed there for the rest of his fifty year life, despite both celebrating Christams every year.

...As for them aging two years, that was simply due to the change in art style shift making all the kids (apart from Timulty) look older rather than any sort of 'time passes here'