xquery and dateTime

Hi, I am using an XQuery Match assertion in one of my tests. I would like to order the XML based on an attribute called StartDate in the XML response. I have found that using a regular text based "order by" is unreliable.

Is it possible to use xs:dateTime in SoapUI? I get an error with the code below. Thanks.

Step 2 [Load_Schedule] FAILED: took 250 ms
-> [XQuery Match] XQuery Match Assertion failed for path [declare namespace soapenv='http://schemas.xmlsoap.org/soap/envelope/';
<Result>
{
for $z in //Foobar
order by xs:dateTime($z/@StartDate)
return $z
}
</Result>] : RuntimeException:Error binding this