How to parse the expiration date from the certificate
Cast it to an X509Certificate and call getNotAfter().
How to determine the certificate chain, eg, the github certificate with chains
You’ve got it. That’s what the Certificate[] array is, as it says in the Javadoc.
How did i know which certificate to get the expiration date from?
Read the Javadoc. “The peer’s own certificate first followed by any certificate authorities”.
However I don’t know why you’re doing any of this. Java should already do it all for you.
And please throw away that insecure and incorrect TrustManager implementation. The correct way to handle self-signed certificates is to import them into the client truststore. Please also throw away your insecure HostnameVerifier, and use the default one, or a secure one. Why use HTTPS at all if you don’t want it to be secure?