Change Time by the use of date and awk command.

change_time.sh

#!/bin/sh
#
# change_time.sh
# original script coded by Takashi Unuma, Kyoto Univ.
# Last modified: 2013/06/21
#

if test $# -lt 3 ; then
    echo "USAGE: $(basename $0) [YYYYMMDDHHNN] [INT] [+/-]"
    echo " *** Note: All values must be 'integer'. *** "
    exit 1
fi

# Input TIME that must be '12 digits' with the format of 'YYYYMMDDHHNN'
INPUTDATE=$1

# time difference, unit is 'second'
INT=$2

# flag must be '+' or '-'
FLAG=$3

# convert from TIME to UNIXTIME
UTIME=$(date +%s --date "${INPUTDATE:0:4}-${INPUTDATE:4:2}-${INPUTDATE:6:2} ${INPUTDATE:8:2}:${INPUTDATE:10:2}")

# Output modified TIME
echo $(expr ${UTIME} ${FLAG} ${INT}) | awk '{print strftime("%Y%m%d%H%M",$1)}'