In the method java.net.HttpURLConnection#getHeaderFieldDate

    public long getHeaderFieldDate(String name, long Default) {
        String dateString = getHeaderField(name);
        try {
            if (!dateString.contains("GMT")) {
                dateString = dateString+" GMT";
            }
            return Date.parse(dateString);
        } catch (Exception e) {
        }
        return Default;
    }

If dateString, expensive NPE is thrown and then caught.
For example it happen with 'last-modified' header.
We should check whether it is present before trying to parse it.