SimpleXMLElement having values stripped out on __soapCall

$domDocument = Hydrate($order);

$client = new SoapClient('http://ift.tt/1odCmhH', array('trace' => 1));

$xml = simplexml_load_string($domDocument->saveHTML());

$result = $client->SendOrder($xml);

Upon debug, $xml ends up having all my values correctly assigned where they should be. Most notably, the DateTime values that I set in the Hydrate() method:

<ns1:OrderDate>2014-08-06T19:37:53+00:00</ns1:OrderDate>

When I SendOrder(), I get:

Server was unable to read request. --> There is an error in XML document (2, 515). --> String was not recognized as a valid DateTime.

And my DateTime values end up being blank:

<ns1:OrderDate/>

I'm beyond frustrated with this. Does anyone know what's going on?