It seems like there may have been a little bit of a slip up at the AMC Theaters website.
As with all movies, theaters need to prepare for the release date, getting booking forms up and running, preparing marketing material and setting up for advanced screenings. AMC Theaters website have set up their booking page for Batman V Superman: Dawn of Justice and it looks like they have given away the runtime of the movie.
The runtime is marked at 2 hours 31 minutes. This is hardly surprising considering the amount of A-List superheroes this movie has. You would be forgiven for expecting it at 3 hours 31 minutes. Having Batman, Superman and Wonder Woman sharing 151 minutes is not going to be easy but it looks like Warner Bros. has pulled it off (we hope so).
The 2 hours 31 minute runtime has not been confirmed or denied at this time so all we can do is speculate.