Custom Un/Marshaling for Non-Standard JSON Time Formats

When dealing with JSON data that contains time values in non-standard formats, the built-in JSON decoder may encounter errors. To handle such situations, custom marshal and unmarshal functions can be implemented.

Consider the following JSON:

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

And the desired Go struct:

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

Using the standard JSON decoder would result in an error while parsing the "birth_date" field. To customize this behavior, a type alias can be created and added to the struct:

type JsonBirthDate time.Time

Then, custom marshal and unmarshal functions are implemented:

func (j *JsonBirthDate) UnmarshalJSON(b []byte) error {
    s := strings.Trim(string(b), `"`) // Remove quotes
    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))
}

With these custom functions, the JSON can now be decoded into the Go struct as intended:

person := Person{}

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

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

// Print the birth date using the Format function
fmt.Println(person.BirthDate.Format("2006-01-02"))