‘Real’ runtime of episode

My addon uses JSON-RPC “VideoLibrary.GetEpisodes” method to get episodes for a TV show and then creates a playlist within the given maximum playing time limit. To track the playing time I use the “runtime” field returned by “VideoLibrary.GetEpisodes”.

Recently I found that the ‘runtime’ field has the same value for all the TV show’s seasons. OK, it works the same as thetvdb.com does. But the point is that some TV shows have ‘Special’ episodes, and its lenght is not the same as the standard episode lenght.

So the question is how can I retrieve the ‘real’ runtime of the episode in my addon?