code: func timeZone2Duration(tz string) time.Duration {
  	sign := 1
  	if strings.HasPrefix(tz, "-") {
  		sign = -1
  	}
  
  	i := strings.Index(tz, ":")
  	h, err := strconv.Atoi(tz[1:i])
  	terror.Log(err)
  	m, err := strconv.Atoi(tz[i+1:])
  	terror.Log(err)
  	return time.Duration(sign) * (time.Duration(h)*time.Hour + time.Duration(m)*time.Minute)
  }
docs: // timeZone2Duration converts timezone whose format should satisfy the regular condition
  // `(^(+|-)(0?[0-9]|1[0-2]):[0-5]?\d$)|(^+13:00$)` to time.Duration.