This bookmarklet calculates the duration of your currently-loaded Now Playing tab. It does this by looking at all of their duration
elements on the page, parsing them, summing them, and then breaking that up into hours, minutes, and seconds, which it adds to the top bar (secondaryMetadata).
Usage is simple. Create a bookmark with a custom URL, and add the code from duration_bookmarklet.js
to it as the URL. Name the bookmark whatever you want and when you want to see your Now Playing duration, assuming you are on the Now Playing page and have the entire playlist loaded, you can click on the bookmark to have it calculated for you.