Google calendar timeline will get the data from your calendar and display it on a separate timeline. Data is NOT saved locally, so you need to be online to get the data.
To add Google calendar timeline, open Timeline editor, then click Add timeline button, choose Google calendar and click Next.
Next you need to provide url to your calendar feed. With your browser navigate to Google calendar, then click on Calendar settings. Copy the link to your XML feed. Paste the url to URL field and click Test connection. You can also set timeline title, and a Refresh rate (how often will ManicTime refresh the data from Google Calendar). Click Ok, then click Ok again to confirm the changes to timelines. By now new Google Calendar timeline should be seen on the Day view.