The registered version of the AuthSub authentication method requires that a signature be added to the Authorization header of your request. The signature can only be added if you have registered your application with Google and have received an e-mail confirming that your registration has been processed.
The signature will always be considered invalid if the time is incorrect on the server you're using to generate the Authorization header. Please verify the time on your server with a trusted time source such as time.gov. Most production servers synchronize their system clock with Network Time Protocol (NTP) servers.