403 Forbidden error when making a push request to APNS with Go. Am I creating the JWT correctly?

I'm getting the following error when making a push notification request to APNS: &{403 Forbidden 403 HTTP/2.0 2 0 map[Apns-Id:[7BF53274-A04B-7C1B-8312-DB01DA48A519]] {0xc0000c88c0} -1 [] false false map[] 0xc000132000 0xc0000ba370}

I suspect my code to generate an authentication header is not correct

Here's the relevant code:

func generateAuthenticationHeader() string {
    b, _ := json.Marshal(Header{ ALG: "ES256", KID: authKeyId })
    header := b64.StdEncoding.EncodeToString(b)

    b, _ = json.Marshal(Claims{ ISS: teamId, IAT: time.Now().String() })
    claims := b64.StdEncoding.EncodeToString(b)

    pkey := getPrivateKey()

    indata := fmt.Sprintf("%s.%s", header, claims)
    h := sha256.New()
    h.Write([]byte(indata))
    digest := h.Sum(nil)

    r, s, err := ecdsa.Sign(rand.Reader, pkey, digest)
    if err != nil {
        log.Fatal(err)
    }

    signature := r.Bytes()
    signature = append(signature, s.Bytes()...)

    signed := b64.StdEncoding.EncodeToString(signature)

    return fmt.Sprintf("%s.%s.%s", header, claims, signed)
}

func getPrivateKey() *ecdsa.PrivateKey {
    r, _ := ioutil.ReadFile(authKeyPath)
    block, _ := pem.Decode(r)

    key, err := x509.ParsePKCS8PrivateKey(block.Bytes)
    if err != nil {
        log.Fatal(err)
    }
    pkey := key.(*ecdsa.PrivateKey)

    return pkey
}

Solution

The problem was incorrect IAT claim type. Instead of string it should be int64 and time should be in seconds. Calling String() on time.Now() returns a different format from what Apple expects the IAT claim to be, per their documentation: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/establishing_a_token-based_connection_to_apns.