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

In Golang, is there a way to make the generic encoding/json Marshal to use a different layout when Marshaling the time.Time fields?

Basically I have this struct:

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

and I want to encoding to json using encdoding/json's Marshal function, but I want to use my custom layout, I imagine somewhere time.Format(layout) is being called, I want to control that layout,

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

http://play.golang.org/p/pUCBUgrjZC

package main

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

type jsonTime struct {
    time.Time
    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 {
            panic(err)
        }
        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.

Maybe something like this will work for you?

package main

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

type jsonTime struct {
t time.Time
f string
}

func (j jsonTime) MarshalText() ([]byte, error) {
return []byte(j.t.Format(j.f)), nil
}

func main() {
x := map[string]interface{}{
	"foo": jsonTime{t: time.Now(), f: time.Kitchen},
	"bar": "baz",
}
data, err := json.Marshal(x)
if err != nil {
	panic(err)
}
fmt.Printf("%s", data)
}

also available here: http://play.golang.org/p/D1kq5KrXQZ

Just make a custom type that implements MarshalText the way you want it to show up.

First, I highly recommend against using a time format other than the default RFC3339. It's a good time format, and can be parsed by any number of languages, so unless you are needing a different format because somebody else's API requires it, it's probably best to use the default.

But, I've had to solve this problem in consuming other people's APIs, so here is one solution that shifts the bulk of the work to the Marshal/Unmarshal step, and leaves you with an ideal structure: http://play.golang.org/p/DKaTbV2Zvl

With generics I found a very hacky way to do this without initializing manually the time fields

type BaseDate[T TimeFormat] struct {
	time.Time
}

func (b *BaseDate[T]) Layout() string {
	format := *new(T)
	return format.String()
}

func (b *BaseDate[T]) Format() string {
	return b.Time.Format(b.Layout())
}

func (b *BaseDate[T]) MarshalJSON() ([]byte, error) {
	//do your serializing here
	stamp := fmt.Sprintf("\"%s\"", b.Layout())
	return []byte(stamp), nil
}

func (b *BaseDate[T]) UnmarshalJSON(data []byte) error {
	// Ignore null, like in the main JSON package.
	str := strings.Trim(string(data), "\"")
	if str == "null" || str == "" {
		return nil
	}
	tt, err := time.Parse(b.Layout(), str)
	b.Time = tt
	return err
}

func Now[T TimeFormat]() BaseDate[T] {
	return BaseDate[T]{Time: time.Now()}
}

And Time format is defined as follow

type TimeFormat interface {
	Datetime | Datehour | Date | Hour
	String() string
}

type Datetime string
type Datehour string
type Date string
type Hour string

const (
	dateTime Datetime = "2006-01-02T15:04:05"
	dateHour Datehour = "2006-01-02T15:04"
	date     Date     = "2006-01-02"
	hour Hour = "15:04"
)

func (d Datetime) String() string {
	return string(DateTime)
}

func (d Datehour) String() string {
	return string(DateHour)
}
func (d Date) String() string {
	return string(date)
}
func (d Hour) String() string {
	return string(hour)
}

I think the function String is unnecessary but add clarity.
This works because I can define the zero value for the type Datetime, Datehour, Date and Hour.