package main

import (
	"encoding/json"
	"fmt"
	"strings"
	"time"
)

const messageTimeLayout = "2006/01/02 15:04:05 -0700"

type MessageTime struct {
	time.Time
}

func (mt MessageTime) MarshalJSON() ([]byte, error) {
	return []byte(fmt.Sprintf(`"%s"`, mt.Time.Format(messageTimeLayout))), nil
}

func (mt *MessageTime) UnmarshalJSON(b []byte) (err error) {
	s := strings.Trim(string(b), `"`)
	if s == "" || s == "null" {
		mt.Time = time.Time{}
	} else {
		mt.Time, err = time.Parse(messageTimeLayout, s)
	}
	return
}

func (mt MessageTime) String() string {
	return fmt.Sprintf(`"%s"`, mt.Time.Format(messageTimeLayout))
}

type Message struct {
	Text      string      `json:"text"`
	CreatedAt MessageTime `json:"created_at"`
}

func main() {
	rawJSON := `{"text":"Hello","created_at":"2018/04/16 23:30:00 +0700"}`
	message := Message{}
	json.Unmarshal([]byte(rawJSON), &message)
	toJSON, _ := json.Marshal(message)
	fmt.Println(string(toJSON))
}

// Output: {"text":"Hello","created_at":"2018/04/16 23:30:00 +0700"}