@Override
    public Long startTimeMs() {
        return startTimeMs;
    }
When the credential became valid, in terms of the number of milliseconds
since the epoch, if known, otherwise null. An expiring credential may not
necessarily indicate when it was created -- just when it expires -- so we
need to support a null return value here.

@return the time when the credential became valid, in terms of the number of
        milliseconds since the epoch, if known, otherwise null
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/BasicOAuthBearerToken.java