'Check HTTPS Certificate' is a very simple addon to check expiration date of your HTTPS certificate. Just drop 'Cert Validity' macro to your Confluence page and you will see something similar to:
You can specify port number in your macro settings, if your server is running on something different than standard https port 443.
You may also use REST API to get check results as JSON:
If your certificate expiration date is more than one week ahead, the macro background will be green; if more than three days – yellow; if three days or less (or already expired) – red.
Hope you find it useful.
This addon is free under MIT license
Source code is available on GitHub