Countdown to the expiry of the DST Root CA X3 certificate

Update: DST Root CA X3 has now expired.
On Sep 30 14:01:15 2021 GMT a root certificate formerly called DST Root CA X3 has expired. Root certificates are important, because they are responsible for anchoring trust in certificates. This root certificate is of interest for many people, because it is one of the three root certificates used by Let's Encrypt. For more information about what this all means, please see Let's Encrypts official announcement and forum entries.
DST Root CA X3 will remain in use, even after it has expired, but with changed compatibility.
DST Root CA X3 has expired!