Comment 10 for bug 1928648

# gnutls-cli --x509cafile=ca.pem expired-root-ca-test.germancoding.com
Processed 2 CA certificate(s).
Resolving 'expired-root-ca-test.germancoding.com'...
Connecting to '2a01:4f8:151:506c::2:443'...
...
- Status: The certificate is NOT trusted. The certificate chain uses expired certificate.
*** PKI verification of server certificate failed...
*** Fatal error: Error in the certificate.
*** Handshake has failed
GnuTLS error: Error in the certificate.

# faketime 2021-10-01 gnutls-cli canonical.com
Processed 129 CA certificate(s).
Resolving 'canonical.com'...
Connecting to '2001:67c:1360:8001::2c:443'...
...
- Status: The certificate is NOT trusted. The certificate chain uses expired certificate.
*** PKI verification of server certificate failed...
*** Fatal error: Error in the certificate.
*** Handshake has failed
GnuTLS error: Error in the certificate.

Reproduced original problem, upgraded libgnutls30 and gnutls-bin

# dpkg-query -W libgnutls30 gnutls-bin
gnutls-bin 3.4.10-4ubuntu1.9
libgnutls30:amd64 3.4.10-4ubuntu1.9

Things are good now:

# gnutls-cli --x509cafile=ca.pem expired-root-ca-test.germancoding.com
Processed 2 CA certificate(s).
Resolving 'expired-root-ca-test.germancoding.com'...
Connecting to '2a01:4f8:151:506c::2:443'...
...
- Status: The certificate is trusted.
- Description: (TLS1.2)-(ECDHE-RSA-SECP256R1)-(AES-256-GCM)
- Session ID: 53:9C:C7:86:1B:69:79:FC:37:AD:AD:A5:82:11:46:84:4F:B4:46:DC:C1:E7:2E:A9:40:18:6C:8A:B9:4C:B9:7E
- Ephemeral EC Diffie-Hellman parameters
 - Using curve: SECP256R1
 - Curve size: 256 bits
- Version: TLS1.2
- Key Exchange: ECDHE-RSA
- Server Signature: RSA-SHA256
- Cipher: AES-256-GCM
- MAC: AEAD
- Compression: NULL
- Options: extended master secret, safe renegotiation, OCSP status request,
- Handshake was completed

# faketime 2021-10-01 gnutls-cli canonical.com
Processed 129 CA certificate(s).
Resolving 'canonical.com'...
Connecting to '2001:67c:1360:8001::2b:443'...
- Certificate type: X.509
- Got a certificate list of 3 certificates.
...
- Status: The certificate is trusted.
- Description: (TLS1.2)-(ECDHE-RSA-SECP256R1)-(AES-256-GCM)
- Session ID: 71:90:E2:22:86:03:D2:13:17:6E:F1:20:8A:57:B8:E9:FF:0E:07:AB:1E:61:F6:7F:56:43:EF:BF:7A:F3:EF:56
- Ephemeral EC Diffie-Hellman parameters
 - Using curve: SECP256R1
 - Curve size: 256 bits
- Version: TLS1.2
- Key Exchange: ECDHE-RSA
- Server Signature: RSA-SHA256
- Cipher: AES-256-GCM
- MAC: AEAD
- Compression: NULL
- Options: extended master secret, safe renegotiation,
- Handshake was completed

All is good. Test website connectivity works, and future connectivity to canonical.com also works.

xenial is verified.