Use start timestamp and duration:
ffmpeg -i "inupt.avi" -ss 00:20:15 -t 12 -c copy "output.avi"
A video clip of 12 seconds will be created.
Or more often, we prefer using start and end timestamp:
ffmpeg -i "inupt.avi" -ss 00:20:15 -to 00:20:27 -c copy "output.avi"