code: func (mc MongoClient) ReadingsByCreationTime(start, end int64, limit int) ([]contract.Reading, error) {
  	return mapReadings(mc.getReadingsLimit(bson.M{"created": bson.M{"$gte": start, "$lte": end}}, limit))
  }
docs: // Return a list of readings whose creation time is in-between start and end
  // Limit by the limit parameter