Dredd's introductory monologue mentions that Mega-City One is "800 million citizens living in the ruins of the old world" - so we can surmise that the setting is pre-Apocalypse.
I don't see any reason why we can't slot the story into Dredd's comic-book timeline - he was already a twenty-year man when the comic first started, and of course he doesn't only fight crime on Wednesdays. He has a lot of "unseen adventures", and the movie can be one of them.