Custom unmarshal

main.go

package main

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

type DateCreatedAt time.Time

type Message struct {
	User          string        `json:"user"`
	DateCreatedAt DateCreatedAt `json:"date_created"`
	Body          string        `json:"body"`
}

const ctLayout = "2006-01-02 15:04:05Z07:00"

func (j *DateCreatedAt) UnmarshalJSON(b []byte) error {
	s := strings.Trim(string(b), `"`)
	t, err := time.Parse(ctLayout, s)
	if err != nil {
		return err
	}
	*j = DateCreatedAt(t)
	return nil
}

// MarshalJSON writes a quoted string in the custom format
func (j DateCreatedAt) MarshalJSON() ([]byte, error) {
	return []byte(j.String()), nil
}

// Maybe a Format function for printing your date
// String returns the time in the custom format
func (j *DateCreatedAt) String() string {
	t := time.Time(*j)
	return fmt.Sprintf("%q", t.Format(ctLayout))
}

func main() {
	var msg Message
	jsonData := `{"user": "d0cc39e3-a976-5dbe-869f-87283d7e3e1c", "date_created": "2020-12-17 19:36:50+00:00", "body": "The content"}`
	json.Unmarshal([]byte(jsonData), &msg)
	//----- output ---------
	//message: "2020-12-17 19:36:50Z"
	//------------------
	fmt.Printf("message: %+v", msg.DateCreatedAt.String())
}