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)
}