How to parse non standard time format from json

lets say i have the following json

{
    name: "John",
    birth_date: "1996-10-07"
}

and i want to decode it into the following structure

type Person struct {
    Name string `json:"name"`
    BirthDate time.Time `json:"birth_date"`
}

like this

person := Person{}

decoder := json.NewDecoder(req.Body);

if err := decoder.Decode(&person); err != nil {
    log.Println(err)
}

which gives me the error parsing time ""1996-10-07"" as ""2006-01-02T15:04:05Z07:00"": cannot parse """ as "T"

if i were to parse it manually i would do it like this

t, err := time.Parse("2006-01-02", "1996-10-07")

but when the time value is from a json string how do i get the decoder to parse it in the above format?

That's a case when you need to implement custom marshal and unmarshal functions.

UnmarshalJSON(b []byte) error { ... }

MarshalJSON() ([]byte, error) { ... }

By following the example in the Golang documentation of json package you get something like:

// First create a type alias
type JsonBirthDate time.Time

// Add that to your struct
type Person struct {
    Name string `json:"name"`
    BirthDate JsonBirthDate `json:"birth_date"`
}

// Implement Marshaler and Unmarshaler interface
func (j *JsonBirthDate) UnmarshalJSON(b []byte) error {
    s := strings.Trim(string(b), "\"")
    t, err := time.Parse("2006-01-02", s)
    if err != nil {
        return err
    }
    *j = JsonBirthDate(t)
    return nil
}
    
func (j JsonBirthDate) MarshalJSON() ([]byte, error) {
    return json.Marshal(time.Time(j))
}

// Maybe a Format function for printing your date
func (j JsonBirthDate) Format(s string) string {
    t := time.Time(j)
    return t.Format(s)
}

If there are lots of struct and you just implement custom marshal und unmarshal functions, that's a lot of work to do so. You can use another lib instead，such as a json-iterator extension jsontime:

import "github.com/liamylian/jsontime"

var json = jsontime.ConfigWithCustomTimeFormat

type Book struct {
    Id        int           `json:"id"`
    UpdatedAt *time.Time    `json:"updated_at" time_format:"sql_date" time_utc:"true"`
    CreatedAt time.Time     `json:"created_at" time_format:"sql_datetime" time_location:"UTC"`
}

I wrote a package for handling yyyy-MM-dd and yyyy-MM-ddThh:mm:ss dates at https://github.com/a-h/date

It uses the type alias approach in the answer above, then implements the MarshalJSON and UnmarshalJSON functions with a few alterations.

// MarshalJSON outputs JSON.
func (d YYYYMMDD) MarshalJSON() ([]byte, error) {
	return []byte("\"" + time.Time(d).Format(formatStringYYYYMMDD) + "\""), nil
}

// UnmarshalJSON handles incoming JSON.
func (d *YYYYMMDD) UnmarshalJSON(b []byte) (err error) {
	if err = checkJSONYYYYMMDD(string(b)); err != nil {
		return
	}
	t, err := time.ParseInLocation(parseJSONYYYYMMDD, string(b), time.UTC)
	if err != nil {
		return
	}
	*d = YYYYMMDD(t)
	return
}

It's important to parse in the correct timezone. My code assumes UTC, but you may wish to use the computer's timezone for some reason.

I also found that solutions which involved using the time.Parse function leaked Go's internal mechanisms as an error message which clients didn't find helpful, for example: cannot parse "sdfdf-01-01" as "2006". That's only useful if you know that the server is written in Go, and that 2006 is the example date format, so I put in more readable error messages.

I also implemented the Stringer interface so that it gets pretty printed in log or debug messages.

Custom implementation of marshal, unmarshal and string methods.

package json
import (
"fmt"
"strings"
"time"
)
const rfc3339 string = "2006-01-02"
// Date represents a date without a time component, encoded as a string
// in the "YYYY-MM-DD" format.
type Date struct {
Year  int
Month time.Month
Day   int
}
// UnmarshalJSON implements json.Unmarshaler inferface.
func (d *Date) UnmarshalJSON(b []byte) error {
t, err := time.Parse(rfc3339, strings.Trim(string(b), `"`))
if err != nil {
return err
}
d.Year, d.Month, d.Day = t.Date()
return nil
}
// MarshalJSON implements json.Marshaler interface.
func (d Date) MarshalJSON() ([]byte, error) {
s := fmt.Sprintf(`"%04d-%02d-%02d"`, d.Year, d.Month, d.Day)
return []byte(s), nil
}
// String defines a string representation.
// It will be called automatically when you try to convert struct instance
// to a string.
func (d Date) String() string {
return fmt.Sprintf("%04d-%02d-%02d", d.Year, d.Month, d.Day)
}

And tests for them.

package json
import (
"encoding/json"
"testing"
"time"
)
func TestDate_UnmarshalJSON(t *testing.T) {
in := `"2022-12-31"`
want := time.Date(2022, time.December, 31, 0, 0, 0, 0, time.UTC)
var got Date
if err := got.UnmarshalJSON([]byte(in)); err != nil {
t.Fatalf("unexpected error: %v", err)
}
if !(got.Year == want.Year() && got.Month == want.Month() && got.Day == want.Day()) {
t.Errorf("got date = %s, want %s", got, want)
}
}
func TestDate_UnmarshalJSON_badFormat(t *testing.T) {
in := `"31 Dec 22"`
var got Date
err := got.UnmarshalJSON([]byte(in))
if err, ok := err.(*time.ParseError); !ok {
t.Errorf("expected a time parse error, got: %v", err)
}
}
func TestDate_MarshalJSON(t *testing.T) {
testcases := map[string]struct {
in   Date
want string
}{
"without zero padding": {
in:   Date{2022, time.December, 31},
want: `"2022-12-31"`,
},
"with zero padding": {
in:   Date{2022, time.July, 1},
want: `"2022-07-01"`,
},
"initial value": {
in:   Date{},
want: `"0000-00-00"`,
},
}
for name, tc := range testcases {
t.Run(name, func(t *testing.T) {
got, err := json.Marshal(tc.in)
if err != nil {
t.Fatalf("unexpected error: %v", err)
}
if string(got) != tc.want {
t.Errorf("got date = %s, want %s", got, tc.want)
}
})
}
}
func TestDate_String(t *testing.T) {
testcases := map[string]struct {
in   Date
want string
}{
"without zero padding": {
in:   Date{2022, time.December, 31},
want: "2022-12-31",
},
"with zero padding": {
in:   Date{2022, time.July, 1},
want: "2022-07-01",
},
"initial value": {
in:   Date{},
want: "0000-00-00",
},
}
for name, tc := range testcases {
t.Run(name, func(t *testing.T) {
if got := tc.in.String(); got != tc.want {
t.Errorf("got %q, want %q", got, tc.want)
}
})
}
}