Under your iDoneThis calendar are three links: Share, Feed, and Invite. Click on Feed and turn on the secret link. You will be given a link, and this is what you feed into Google Calendar and iCal. Now just follow the Google Calendar instructions or the iCal instructions to feed your iDoneThis into them.
Please note that this option is currently only available for personal calendars. If you wish to feed a team calendar into your google calendar, please shoot us an email and we'll be happy to help!
Need more help? email us at firstname.lastname@example.org