json: error calling MarshalJSON for type main.Time: invalid character '-' after top-level value

I intent to serialize time.Time to have UTC timezone and in RFC3339 format.
So I created following custom data type called Time that embeds time.Time.

package main

import (
	"encoding/json"
	"time"
)

type Time struct {
	time.Time
}

func (t *Time) UnmarshalJSON(b []byte) error {
	ret, err := time.Parse(time.RFC3339, string(b))
	if err != nil {
		return err
	}
	*t = Time{ret}
	return nil
}

func (t Time) MarshalJSON() ([]byte, error) {
	return []byte(t.UTC().Format(time.RFC3339)), nil
}

func main() {
	type User struct {
		CreatedAt Time `json:"created_at"`
	}

	user := User{CreatedAt: Time{time.Now()}}
	_, err := json.Marshal(user)
	if err != nil {
		panic(err)
	}
}

When this data is tried to marshal with json.Marshal I get following error

json: error calling MarshalJSON for type main.Time: invalid character '-' after top-level value

I have 2 queries

1. Why above code is throwing the error?
2. Is this the right way to serialise and deserialise time.Time to and from a desired format in an API

Thanks in advance!

The text 2009-11-10T23:00:00Z is not a valid JSON value, but "2009-11-10T23:00:00Z" is. Note the quotes.

Use the following function to format Time as a JSON string.

func (t Time) MarshalJSON() ([]byte, error) {
	b := make([]byte, 0, len(time.RFC3339)+2)
	b = append(b, '"')
	b = t.AppendFormat(b, time.RFC3339)
	b = append(b, '"')
	return b, nil
}