code: func (b *Backend) setExpirationTime(key string) error {
  	expiresIn := b.GetConfig().ResultsExpireIn
  	if expiresIn == 0 {
  		// // expire results after 1 hour by default
  		expiresIn = config.DefaultResultsExpireIn
  	}
  	expirationTimestamp := int32(time.Now().Unix() + int64(expiresIn))
  
  	conn := b.open()
  	defer conn.Close()
  
  	_, err := conn.Do("EXPIREAT", key, expirationTimestamp)
  	if err != nil {
  		return err
  	}
  
  	return nil
  }
docs: // setExpirationTime sets expiration timestamp on a stored task state