[jis]

Sorry to reply to my own comment. But I want to clarify. Two certificates (at least) expired. The root named "AddTrust External CA Root" and a subordinate certificate with a subject of "USERTrust RSA Certification Authority." Both expired around the same time.

The "USERTrust RSA Certification Authority" certificate signed yet another layer of intermediate certificates.

The "USERTrust RSA Certification Authority" certificate was promoted to a self-signed certificate, now in the browser trust stores, using the same key pair as the original certificate that was signed by "AddTrust External CA Root." It has an expiration of 2038 (although that concept is a bit vague in a root certificate).

[josephcsible]

There's actually a third certificate for "USERTrust RSA Certification Authority", also using the same key pair, signed by a different root called "AAA Certificate Services". It looks like the intended replacement for the expiring one is this one, rather than the one where it's the root itself.

[dylz]

It is explicitly not a replacement, but some kind of legacy fallback that they don't want you to use, but exists for enterprise customers that absolutely can't get trust.