Have a read through the bug report above (which I have now closed).
Do you have the following set in your windows inventory/ group vars:
ansible_winrm_server_cert_validation: ignore
as described in;
http://docs.ansible.com/ansible/intro_windows.html#inventoryIf so the certificate is completely ignored. Traffic between the ansible node and the windows winrm endpoint is still encrypted, but ansible doesn't check anything to do with the certificate. So I think in your case the certificate really has expired but the certificate is not getting checked.
It would be good to have a documented way to retrieve the generated certificate from the windows host and add it to the trusted certs on the ansible controller, but I don't believe we have that right now. It might be tedious and error prone if it requires manual steps on each host of course so would be best automated.
Depending on how you set things up, certificates aren't the only way in which you can lock down access to a time period on windows hosts. If you set up domain authentication then you can configure quite a short period for the kerberos tickets to last (I think that the default is 10 hours without renewal).
Hope this helps
Jon