http://youtube-playlist-analyzer.appspot.com/ from https://www.reddit.com/r/math/comments/4r9dkm/is_there_a_good_online_calculator_to_add_total/d4zb2te :

"YouTube Playlist Analyzer uses YouTube Data API to analyze any youtube playlist or channel, and display the related information graphically. It supports 3 modes of data representations (or views): Playlist View, Timeline View and Table View. Use this tool to find out things like follows:

Total Duration of a playlist?
Compare video durations in a playlist/channel.
What is the trend of views, likes, dislikes or comments on videos of a channel/playlist?
Details of each video item - title, channel title, duration, counts, publishing time, short description, video link."

Its total playlist times could be useful for YouTube's video playlist submissions, @dag and @lucky760!