#-ss option to specify a start timestamp, and the -t option to specify the encoding duration.
# The timestamps need to be in HH:MM:SS.xxx format or in seconds (s.msec).
ffmpeg -ss 00:00:30.0 -i input.wmv -c copy -t 00:00:10.0 output.wmv
ffmpeg -ss 30 -i input.wmv -c copy -t 10 output.wmv