type ProfileJSON struct {
	Name      string `json:"name"`
	Address   string `json:"address"`
	CreatedAt int64  `json:"created_at"`
}

type Profile struct {
	Name      string
	Address   string
	Password  string
	CreatedAt string
}

func (p *Profile) MarshalJSON() ([]byte, error) {
	millis, err := time.Parse(time.RFC3339, p.CreatedAt)
	if err != nil {
		return nil, err
	}

	res := ProfileJSON{
		Name:      p.Name,
		Address:   p.Address,
		CreatedAt: millis.UnixNano() / 1000000,
	}

	return json.Marshal(&res)
}

func main() {
	profile := Profile{
		Name:      "name",
		Address:   "address",
		Password:  "password",
		CreatedAt: "2019-01-22T06:11:43Z",
	}

	res, err := json.Marshal(&profile)
	if err != nil {
		fmt.Println(err)
	}

	fmt.Println(string(res))
}