Your Gmail address:
1. Navigate to the Google Developers Console
2. From the top Project menu select Create project.
3. Give your project a name, agree to the terms, then click Create.
4. From the Google Developers Console Dashboard select Enable API. If you don't see this, select API Manager from the top-left 'hamburger' menu.
5. Under Google Apps APIs, select Calendar API. Then click Click Enable.
6. Select Credentials under API Manager in the left-hand menu, click Create credentials, then select API key. You can get to the same result using the Go to Credentials button, but the screens you are taken through are a bit different.
7. On the API key created popup, select and copy (Cmd-C or Ctrl-C) your newly created API key.
Remember that your calendar settings has to be set to public if you want to display it on your mirror!