There are a few key issues with your code, I'll outline them briefly.
You are not properly constructing the extended SoapClient.
return new SoapClient(...) should be
parent::__construct(...). Constructor methods should never
The next issue, and this is the big one, the EarthTools web services (docs) do not talk SOAP. They're plain XML that you need to figure out how to a) construct the correct URLs for your needs (looks like you can do this already), and b) parse the response XML into PHP variables yourself.
For part b), there are lots of tools available including DOM, SimpleXML and XMLReader. See http://php.net/refs.xml
A very basic example would look like:
$url = 'http://www.earthtools.org/timezone/'.urlencode($lat).'/'.urlencode($lon);
$xml = simplexml_load_file($url);
// Then access the values
$localtime = (string) $xml->isotime;
echo $localtime; // e.g. 2011-09-21 07:09:50 -0500