The clearest solution:
#!/bin/bash
function calc_date(){
odate=$(date -d "$1" +%d%m%y)
op=$2
days=$3
echo $(date "--date=$odate $op $days day" +%d%m%y)
}
Example:
mydate=$(calc_date '220214' '-' '2')
echo "$mydate"
mydate=$(calc_date '220214' '+' '3')
echo "$mydate"
Result:
200214
250214