Custom UnmarshalJSON for custom time format returning this {0 63082400645 <nil>} instead of time with ShouldBindJSON from Gin gonic

I have this type and handler

type Bday time.Time

func (mt *Bday) UnmarshalJSON(bs []byte) (err error) {
	var t time.Time
	layout := "2006-01-02T15:04:05Z0700" // RFC3339
	s := strings.Trim(string(bs), `"`)

	t, err = time.Parse(layout, s)
	if err != nil {
		return err
	}

	*mt = Bday(t)

	return
}

func (mt Bday) MarshalJSON() ([]byte, error) {
	timestamp := time.Time(mt)
	return json.Marshal(timestamp)
}

type LoginHandler struct {
	db *gorm.DB
}

func (l *LoginHandler) Signup(c *gin.Context) {
	// Verify name, lastName, email, password, gender, birthDate
	var user User


	if err := c.ShouldBindJSON(&user); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(user.Password), 8)
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	user.Password = string(hashedPassword)
	fmt.Println("User", user)
	// Save to db
	l.db.Debug().Model(&models.User{}).Create(&user)

	// Return code 201
	c.JSON(http.StatusCreated, gin.H{"message": "User created successfully!"})
}

This is the User

type User struct {
Password  string    `json:"password"`
Email     string    `json:"email"`
Name      string    `json:"name"`
LastName  string    `json:"last_name"`
Bday      Bday      `json:"bday"`
Gender    string    `json:"gender"`
CreatedAt time.Time `json:"created_at"`
UpdatedAt time.Time `json:"updated_at"`
DeletedAt time.Time `json:"deleted_at"`
}

When I print User in the handler I get this for the Bday field

{0 63082400645 <nil>}

This is the full println

User {$2a$08$uTE7Llc6YM2owu.D6UWqpOcOVFHRFRrSxbkxWV4ecR72G6kK5/SNq d.n.m@gmail.com
Antonio Magalhães {0 63082400645 <nil>} M 0001-01-01 00:00:00 +0000 UTC 0001-01-01
00:00:00 +0000 UTC 0001-01-01 00:00:00 +0000 UTC}

What I expected whas that Bday were something like a date
2001-01-01 00:00:00 +0000 UTC

Can somebody help me understand this?

Your custom type Bday satisfies neither Formatter nor Stringer interface, and thus gets printed as a compound struct (the fields of the underlying time.Time struct are printed individually).

An easy fix would be to implement a String() method, for example delegating to MarshalJSON:

func (mt Bday) String() string {
b, _ := mt.MarshalJSON()
return string(b)
}

For more details on how fmt.Println works refer to fmt package documentation.