public static function getStringValue($dateTime): string
    {
        // do not return anything if no value was specified
        if (!$dateTime) {
            return '';
        }

        // until a mixed type is supported for type-hints, check the value here
        if (!$dateTime instanceof \DateTime) {
            return (new \DateTime($dateTime))->format(static::DATE_FORMAT);
        }

        return $dateTime->format(static::DATE_FORMAT);
    }