public Number issuedAt() throws OAuthBearerIllegalTokenException {
        return claim("iat", Number.class);
    }
Return the <a href="https://tools.ietf.org/html/rfc7519#section-4.1.6">Issued
At</a> claim

@return the
        <a href= "https://tools.ietf.org/html/rfc7519#section-4.1.6">Issued
        At</a> claim if available, otherwise null
@throws OAuthBearerIllegalTokenException
            if the claim value is the incorrect type
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/unsecured/OAuthBearerUnsecuredJws.java