#!/bin/sh
t="2018-12-30T08:36:50+09:00"
IFS="-T:+"
set -- $t
case ${t%?????} in
*+) echo $1 $2 $3 $4 $5 $6 + $7 $8 ;;
*-) echo $1 $2 $3 $4 $5 $6 - $7 $8 ;;
esac