UTC datetime problem with nusoap

I am calling a SOAP web service using the nusoap class library, I am able to call the web method correctly however 1 of the parameters that i must pass in is a datetime in UTC format. i can create the correct format and time in my client code.
eg. 2008-04-11T10:00:25 00:00
however when i pass this datetime into the soap method nusoap alters the datetime to the one based on my local machine (BST).
A simplified version of the code i am using is:

//create date in correct format 2008-03-17 14:29:36Z
$date = new DateTime();

$params = array(
'Username' => $username,
'RequestDateTime' => $date->format(DATE_RFC3339),
'ClientId' => $clientId,

$soapclient = new soapclient($wsdl,true);

$proxy = $soapclient->getProxy();
$result = $proxy->ValidateFirstFacPassword($params);

All other variables are sent unaltered.
Thanks in advance Nigel.

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