will remove the type, and i can not answer the "why". Ther is a decission in the `getSubscribingMethods()`
```
        foreach (array('json', 'xml', 'yml') as $format) {
            $methods[] = array(
                'type' => 'DateTime',
                'direction' => GraphNavigator::DIRECTION_DESERIALIZATION,
                'format' => $format,
            );
            //the DateInerval is no able to be deserialzed from xml
            if($format == "xml"){
                $methods[] = array(
                    'type' => 'DateInterval',
                    'format' => $format,
                    'direction' => GraphNavigator::DIRECTION_DESERIALIZATION,
                    'method' => 'deserializeDateIntervalXml',
                );
            }
            foreach ($types as $type) {
                $methods[] = array(
                    'type' => $type,
                    'format' => $format,
                    'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
                    'method' => 'serialize'.$type,
                );
            }
        }
```
where i added the support for it.
I know there should be a way do do it for all formats.