I guess this goes under this topic? Anyway, I tried to access a site and i was greeted with "your connection is not private" error. I'm sure you've all heard of it. Anyway, I did a clean of my computer to get rid of any malware blocking the site, cleaned my browser, changed the date and time which didn't work and even if it did i want to use the real date, and tried to edit the proxy settings in CP whilst following a YouTube tut but the settings were already correct. I also have no anti virus installed so that couldn't be blocking it.
You shouldn't proceed to website unless you can actually figure out what is happening, If you are being presented with invalid HTTPS certificates malicious actors or man in the middle attacks could be reading all the data you are sending including usernames, passwords and credit card info.
Make sure the date and time of your own computer is correct (Sounds like you already did)
Reboot the system if say the website XYZ.com certificate expired, and they renewed it but didnt close your browser or reboot the old certificate is still cached and it will use the old one which can be invalid.
When you browse to said website you can view the certificate, encryption and hashing method along with the expiration date.