How do I crop (time not picture) an MP4 video file

ffmpeg

ffmpeg -i source.mp4 -ss 00:00:00 -t 00:00:00 -vcodec copy -acodec copy splice.mp4

This will take source.mp4 and, using the same audio and video codecs and not degrading quality, take -t time after -ss start time video and output your splice.mp4

Please note that the timing is very sensitive to formatting (above is one example formatting). Also, in some distributions, ffmpeg has been replaced with avconv. Simply run the above command and replace the word ffmpeg with avconv to solve this.

Another excellent source of information on this topic. (Thanks @pbhj)

In Ubuntu 16.04 the equivalent of ffmpeg is the command avconv

To install it run:

sudo apt-get install libav-tools

Example command line to crop a video by time:

avconv -i originalFile.ogv -ss 00:00:00 -t 00:00:10 -vcodec copy -acodec copy croppedFile.ogv