(admin note: this post and the 2 following ones have been moved here from this thread)
We now have an ffmpeg thread which works on Mac, Linux and probably PC (at least on Windows Subsystem for Linux but possibly also in the Cmd prompt?).
To make a short clip, you can trim a video from a given start time mm:ss to an end time mm2:ss2. You can also convert between formats simply by changing the output file extension.
ffmpeg -ss {{mm:ss}} -to {{mm2:ss2}} -i {{input-video.mp4}} -codec copy {{output.mp4}}