DateTime.Parse(certificate.GetExpirationDateString()) throws parse exception if you're running in certain culturesettings.

From https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.x509certificates.x509certificate.getexpirationdatestring?view=netcore-3.1#remarks
The GetExpirationDateString method returns a string that shows the date formatted in Short Date Pattern followed by the time formatted in Long Time Pattern. The date and time are formatted using the current culture and time zone.

Proposed change that seemed to fix it for my case:

            CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
            DateTime expirationDate = DateTime.Parse(certificate.GetExpirationDateString(), CultureInfo.InvariantCulture);

For reference this issue occured with a CultureInfo of "NL-nl".

Is it possible to extend the check with TLS version to ensure outdated 1.x or others.

Thanks