How can I display the "length" of selected keyframes (number of frames in-between) Sometimes in the Dope Sheet, I want to find out quickly the number of frames it took to get from one pose to another. I'd imagine some kind of tool to let me specify a start frame and then an end frame and it would tell me how long was that in terms of number of frames. But so far it seems there's no such tool. I tried Googling and can't find anything there either.

Does anyone know of such a tool or plugin to let me do this? And if not, can anyone direct to as to which part of the Python API to look into to start getting into creating such a tool?
So I ended up trying to make an add-on by myself. It works, but the code is a mess, since it's been years since I last did anything in Python, and I know little about the Blender Python API. But it works.

Source code is here (MIT license): SelectedKeyframesLength.py