The site is currently presenting a Let's Encrypt certificate issued just under 24 hours ago. There have historically always (for reasonable definitions of always) been valid certs for this site, although of course I can't tell from here that they were always properly installed.
Likely explanations for your experience:
1. Your clock is wrong. If your system currently thinks this is Thursday 11 October for example, that's a problem, 'cos this is Friday 12 October.
2. There's some subtle configuration error on their server (seems unlikely as it looks to be just a generic AWS setup) that results in the wrong certificate being presented.
3. Your OS or browser trust store lacks the root CA "DST Root CA X3" operated by IdenTrust. If you didn't deliberately choose to do this, you should investigate as most likely you aren't getting important security updates.
All three causes can often by diagnosed by closely examining the detailed error reported in a browser e.g. SEC_ERROR_EXPIRED_CERTIFICATE
Likely explanations for your experience:
1. Your clock is wrong. If your system currently thinks this is Thursday 11 October for example, that's a problem, 'cos this is Friday 12 October.
2. There's some subtle configuration error on their server (seems unlikely as it looks to be just a generic AWS setup) that results in the wrong certificate being presented.
3. Your OS or browser trust store lacks the root CA "DST Root CA X3" operated by IdenTrust. If you didn't deliberately choose to do this, you should investigate as most likely you aren't getting important security updates.
All three causes can often by diagnosed by closely examining the detailed error reported in a browser e.g. SEC_ERROR_EXPIRED_CERTIFICATE