I installed the sensor platform cert_expiry this these lines in my sensors.yaml file:
- platform: cert_expiry
host: home-assistant.io
This reports 27 days left on October 20.
However, I happened to notice in the log file for the Duckdns Add On to Hassio that there was an attempt to renew the certificate but this failed because the expiry date is greater than 30 days. Reporting an expiry date of January 7.
It would appear that the cert_expiry sensor is not compatible with the new way of certificate renewal used by the Duckdns add on to hassio.
Maybe I should just relax and trust that this new method will keep me securely connected automatically. So much easier than the Hassbian environment I came from.
Still, It would be nice to be able to monitor the new certificate renewal process.