public function formatDiff(DateTimeInterface $from, DateTimeInterface $to)
    {
        static $units = array(
            'y' => 'year',
            'm' => 'month',
            'd' => 'day',
            'h' => 'hour',
            'i' => 'minute',
            's' => 'second'
        );

        $diff = $to->diff($from);

        foreach ($units as $attribute => $unit) {
            $count = $diff->$attribute;
            if (0 !== $count) {
                return $this->doGetDiffMessage($count, $diff->invert, $unit);
            }
        }

        return $this->getEmptyDiffMessage();
    }