code: func (s *streamImpl) expireTime() (t time.Time, has bool) {
  	s.withParserLock(func() error {
  		t, has = s.c.parser.firstChunkTime()
  		return nil
  	})
  
  	if has {
  		t = t.Add(s.c.maximumBufferDuration)
  	}
  	return
  }
docs: // expireTime returns the Time when the oldest chunk in the stream will expire.
  //
  // This is calculated ask:
  // oldest.Timestamp + stream.maximumBufferDuration
  // If there is no buffered data, oldest will return nil.