gistfile1.txt

#!/bin/bash
#
# Usage:  ffmpeg_cut.sh start-time stop-time file-in file-out
# where:
#  start-time stop-time has the format: hh:mm:ss (hh,mm optional)
#
function cnt_sec () {
  echo $1 | awk -F":" '{ if (NF>2) { sec += 60*60 * $(NF-2) } ;
        if (NF>1) { sec += 60 * $(NF-1) } ;
        if (NF>0) { sec += $NF } ; ## seconds
        print sec }'
}

STRT_TIME=`cnt_sec $1`
STOP_TIME=`cnt_sec $2`

DURATION=$(($STOP_TIME-STRT_TIME))
echo $DURATION

ffmpeg  -ss $1 -t $DURATION -i $3 $4