//get duration between 0:00:00 UTC and start shift by UTC
func getUTCDiffStartShift(loc string, startShift time.Duration) (time.Duration, error) {
    locDiff, err := getLocationDiff(loc)
    if err != nil {
        return time.Duration(0), fmt.Errorf("can not diff between 0:00:00 and start shift: %v", err)
    }
    return startShift - locDiff, nil
}

//get different between UTC and location as duration
func getLocationDiff(loc string) (time.Duration, error) {
    location, err := time.LoadLocation(loc)
    if err != nil {
        return time.Duration(0), err
    }
    in := time.Now().In(location)
    locDiff, err := strconv.ParseFloat(in.Format("-07"), 64)
    if err != nil {
        return time.Duration(0), err
    }
    return time.Duration(locDiff) * time.Hour, nil
}