If we wanted to be able to refer to that date description as described in the uniqueness example, just change it to a key.

This allows other elements in the document to refer to this date value.

The changed declaration:

<xs:element name="description-of-date">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="xhtml:p" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="date" type="xs:date" use="required"/>
  <xs:complexType>
  <xs:key name="date-key">
    <xs:selector xpath="."/>
    <xs:field xpath="@date"/>
  </xs:key>
</xs:element>