Unexpected output when printing time.Time type alias

I am trying to write a unmarshal function for a custom type.
Consider the below code (playground)

package main

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

type Time time.Time

func (st *Time) UnmarshalJSON(b []byte) error {
	// "2021-05-21T03:10:20.958450" -> "2021-05-21T03:10:20.958450Z"
	s := strings.Trim(string(b), "\"")
	t, err := time.Parse(time.RFC3339, fmt.Sprintf("%s%s", s, "Z"))
	if err != nil {
		return fmt.Errorf("parse time: %w", err)
	}
	*st = Time(t)
	return nil
}

type User struct {
	Name string
	TS Time
}

const data = `{"id":3, "name":"Name", "ts":"2021-05-21T03:10:20.958450"}`

func main() {
	user := new(User)
	json.Unmarshal([]byte(data), &user)
	fmt.Printf("%v\n", user)
}

I am successfully able to get a valid time.Time value from my time.Parse() but I am not quite grasping why *st = Time(t) gives such a wierd value?

Currently the above prints out:

&{Name {958450000 63757163420 <nil>}}

But I would like to be more similar to:

&{Name 2021-05-21 03:10:20.95845 +0000 UTC}

What am I missunderstanding here?

Contrary to time.Time your type does not implement fmt.Stringer, so the fmt.Print* functions have no choice but to use their default formatting logic, which in this case is to print the fields of the underlying time.Time value enclosed in curly braces.

Add a String method which delegates to time.Time.String to your type to get the desired behavior:

func (t Time) String() string {
    return time.Time(t).String()
}

https://go.dev/play/p/5PwOwa49B5X

Alternatively, change your Time type to embed time.Time. This will automatically promote the String method along with all other methods (such as the Marshal* methods):

type Time struct {
	time.Time
}

func (st *Time) UnmarshalJSON(b []byte) error {
	// ...
	st.Time = t // simple assignment without type conversion
	return nil
}

https://go.dev/play/p/0H5qyCO22gu

Also, you should never parse JSON by hand. strings.Trim(string(b), "\"") is not enough to fully decode a JSON string value. Always use json.Unmarshal. And you can simplify by using time.ParseInLocation.

func (st *Time) UnmarshalJSON(b []byte) error {
    var s string
    if err := json.Unmarshal(b, &s); err != nil {
        return err
    }

    t, err := time.ParseInLocation("2006-01-02T15:04:05", s, time.UTC)
    if err != nil {
        return err
    }

    // ...
}