NUSOAP datetime UTC issue

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_default_timezone_set('UTC');
                $date = new DateTime();
                       
                $params = array(
                        'Username'         => $username,
                        'RequestDateTime'  => $date->format(DATE_RFC3339),
                        'ClientId'         => $clientId,
                );
       
                $wsdl="http://localHost/twoFac/service.asmx?WSDL";
                $soapclient = new soapclient($wsdl,true);
               
                $proxy = $soapclient->getProxy();
                $result = $proxy->ValidateFirstFacPassword($params);

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