def get_last_modified(self):
return None<EOL>
Contains the Last-Modified header returned by a GET method without
        accept headers.

        Return None, if this live property is not supported.

        Note that the last-modified date on a resource may reflect changes in
        any part of the state of the resource, not necessarily just a change to
        the response to the GET method. For example, a change in a property may
        cause the last-modified date to change. The getlastmodified property
        MUST be defined on any DAV compliant resource that returns the
        Last-Modified header in response to a GET.

        This method SHOULD be implemented, especially by non-collections.