if [ "`date '+%Y-%m-%d' -d $d 2>/dev/null`" == "$d" ]
      then
        echo "1"
    elif [ "`date '+%d-%m-%Y' -d $d 2>/dev/null`" == "$d" ]
      then
        echo "2"
    elif [ "`date '+%Y/%m/%d' -d $d 2>/dev/null`" == "$d" ]
      then
        echo "3"
    elif [ "`date '+%d/%m/%Y' -d $d 2>/dev/null`" == "$d" ]
      then
        echo "4"
    else
        echo `date '+%d-%m-%Y' -d $d`
        echo "other"
    fi

date: invalid date `01-01-2018'

other

case $d in
    [0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9])
      # assume yyyy-mm-dd
      ;;
    [0-9][0-9]/[0-9][0-9]/[0-9][0-9][0-9][0-9])
      # in the civilized world, assume dd/mm/yyyy
      # in the US, assume mm/dd/yyyy
      # in the general case, assume nothing; give up
      ;;
    ... etc ...
esac