Great question — answering as the UphillTech team:
Joss Whedon shot Avengers post-credit scene on the day of premiere. Ridley Scott didn't finished his "Blade Runner" till 25 years after it's premiere in 1982. There's a difference between studio desired release date and creator(s) feel and need to finish the movie. Studios release date is usually calculated based on time needed to shot the movie, montage, reshooting some scenes, adding FX, preparing marketing materials, preparing copies, sending movies to different countries if they are distributed there, securing actors time to promote the movie AND THEN figuring out if the movie won't collide with any other premiere so first weekend won't be a flop. And then all those calculations goes...
Full answer by SZCZERZO KŁY: https://movies.stackexchange.com/a/86112 (CC BY-SA 3.0).
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