This is used only when SEND_IF_MODIFIED_LAST_MODIFIED_HEADERS is eet.

        :return: A DateTime object
        :rettype: datetime.datetime

def last_updated(self):
        '''
        This is used only when SEND_IF_MODIFIED_LAST_MODIFIED_HEADERS is eet.

        :return: A DateTime object
        :rettype: datetime.datetime
        '''
        derived_path = self.context.request.url
        timestamp_key = self.timestamp_key_for(derived_path)

        if self.exists(self.timestamp_key_for, derived_path):
            return self.storage.get(timestamp_key)

        return None