ffmpeg -ss $START -i $INFILE -c copy -map 0 -to $END $OUTFILE This is functionally the same as ffmpeg -ss $START -i $INFILE -c copy -map 0 -t $END $OUTFILE

ffmpeg -ss $START -i $INFILE -c copy -map 0 -to $END $OUTFILE This is functionally the same as ffmpeg -ss $START -i $INFILE -c copy -map 0 -t $END $OUTFILE FFmpeg normalizes input timestamps unless told otherwise via -copyts. So, in the first command, output timestamps start from 0, effectively making -to a duration limiter. Both -ss and -to should be on the same side of the input for (close to) expected result.