#! /bin/bash
DATE=`date +%Y%m%d`
TIME=$1
NEWTIME=`date -d "$DATE $TIME - 3 minutes" +%H%M%S`
echo $NEWTIME

$ ./myscript.sh 123456
invalid date `20141022 123456 - 3 minutes'

TIME=${TIME:0:2}:${TIME:2:2}:${TIME:4:2}

$ ./myscript.sh 123456
103356