Ali Ali - 2 months ago 5x
JSON Question

Override the layout used by json.Marshal to format time.Time

In Golang, is there a way to make the generic

Marshal to use a different layout when Marshaling the

Basically I have this struct:

s := {"starttime":time.Now(), "name":"ali"}

and I want to encoding to json using
function, but I want to use my custom layout, I imagine somewhere
is being called, I want to control that layout,


As inspired by zeebo's answer and hashed out in the comments to that answer:

package main

import "fmt"
import "time"
import "encoding/json"

type jsonTime struct {
    f string

func (j jsonTime) format() string {
    return j.Time.Format(j.f)

func (j jsonTime) MarshalText() ([]byte, error) {
    return []byte(j.format()), nil

func (j jsonTime) MarshalJSON() ([]byte, error) {
    return []byte(`"` + j.format() + `"`), nil

func main() {
    jt := jsonTime{time.Now(), time.Kitchen}
    if jt.Before(time.Now().AddDate(0, 0, 1)) { // 1
        x := map[string]interface{}{
            "foo": jt,
            "bar": "baz",
        data, err := json.Marshal(x)
        if err != nil {
        fmt.Printf("%s", data)

This solution embeds the time.Time into the jsonTime struct. Embedding promotes all of time.Time's methods to the jsonTime struct, allowing their use without explicit type conversion (see // 1).

Embedding a time.Time has the downside of also promoting the MarshalJSON method, which the encoding/json marshaling code prioritizes higher than the MarshalText method for backwards compatibility reasons (MarshalText was added in Go 1.2, MarshalJSON predates that). As a result the default time.Time format is used instead of a custom format provided by MarshalText.

To overcome this problem we override MarshalJSON for the jsonTime struct.