title: XSD Date with attribute but optional value
body: <p>How do I an XSD date value which is optional? Is there a way I could escape from using nillable?</p>
  
  <p>For instance, both</p>
  
  <pre><code>&lt;element attribute="attribute"&gt;optional-value&lt;/element&gt;
  &lt;element attribute="attribute"/&gt;
  </code></pre>
  
  <p>are valid types, where "optional-value" must be defined as an xsd:date type.</p>