I have done so little of this kind of thing that am practically illiterate. Anyone have any suggestions for selecting and saving separately a small portion of mp4 footage from my dashcam? I though videocutter would do the job - but I can’t seem to manage to get the results out (easy enough to choose the section, but saves don’t seem to happen).
This is all I need out of the package - so a command line tool is fine too (if such exist).
And yes, I need the footage for evidentiary purposes 
I use ffmepeg for cutting sections out of a video; I can’t recall it ever failing to work for me.
ffmpeg -i INPUT.mp4 -ss 00:15:35 -t 00:05:21 -vcodec copy -acodec copy OUTPUT1.mp4
WHERE -ss is start time of the cut, and -t is duration of cut. Change INPUT.mp4 to the name of the actual video file, of course.
(The time values in the above command are for example only - this would cut a clip 5min 21sec long from the video, starting at 15min 35sec)
As a U.S. lawyer, I would tell you to be careful with tendering only a part of a video for litigation purposes. In the U.S., the other side has the right to subpoena the full version of the video for impeachment purposes and for showing the total context/ content of the evidence. I don’t know how it works in Canada, but be sure to consult with a local attorney …
FWIW, and good luck!
What @anon3337769 suggested is probably the easiest solution. You can use some GUI program, like Kdenlive, which is very easy to figure out, but if you want it quickly, just use ffmpeg.
I like to put the -i option after -t so that it limits the input file not the output file. It shouldn’t matter most of the time, but just in case.
This should be sufficient:
ffmpeg -ss START_TIME -t DURATION -i INPUT_FILENAME OUTPUT_FILENAME
You are quite correct that legally it would probably need context - probably here too. I would have been happy to send the whole thing in this case, but for size limitations! However - luckily this is a private negotiation, meant to avoid the legal system - and keep costs down.
Thanks for the heads up, though! 
Interesting modification - but I think I’ll survive this time- the input file is only a minute long. The dashcam splits the files into minute-long segments as it runs in continuous record mode…
I would have preferred if the ‘other guy’ had looked in his mirror before changing lanes with his 5-ton straight truck, though. I got MOSTLY out of his way, but even with the horn he didn’t notice me at all, and got my passenger side mirror…
Edit: the video captured the outer skin of the mirror departing vigourously ahead of the car too - and that will make it into the final cut…