docs: 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