I thought I should post this because I could find nothing about it whatsoever, other than a few posts from random server admins who use the same library for other projects. I wasn’t able to play at all until GM Telarusch descended from the sky and bestowed upon me a link to the solution.
If you get the error “server’s certificate is not yet valid” when trying to do anything in the launcher (i.e. log in), simply download the Windows update KB3004394, and restart your PC. That fixed it for me.