<s:element minOccurs="1" maxOccurs="1" name="endDate" nillable="true" type="s:dateTime" />

This is regarded by SoapUI as a mandatory field and must have a valid DateTme entry.

The correct behaviour should be to allow an empty field to be used and if the value is empty (nill) the XML should have a corresponding entry of:

 
                  <endDate xsi:nil="true"/>

A WSDL file that has this property set is attached.