See my notes on getting a root certificate into a const char array.
To see the root certificate expiration date, follow the above instructions to get to the Certificate Viewer | Details window. The top window pane, Certificate Hierarchy, shows the certificate chain with the root certificate at the top and the server certificate at the bottom. Click on the root certificate so it is highlighted.
The middle pane, Certificate Fields, shows the root certificate fields. Click on the field named "Not After" so it is highlighted.
The bottom pane, Field Value, shows the expiration date.
The following screenshot for https://www.google.com shows the root certificate expiration date is May 21, 2022. In contrast, the intermediate and server certificates have expiration dates in 2018.
