If for some reason you want to know when a project was added to CircleCI, you can do the following.
With API auth configured (a token, or no token required via a logged in browser), curl, and jq, you can run the following:
curl "https://circleci.com/api/v1.1/project/github/<org>/<repo>/1" | jq .steps[0].actions[0].start_time
# Replace <org> with your GitHub org and 
# replace <repo> with your project's repository name.
For example, for my person website Feliciano.Tech (which is open source) you can run:
curl "https://circleci.com/api/v1.1/project/github/felicianotech/www.feliciano.tech/1" | jq .steps[0].actions[0].start_time
% Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 20160  100 20160    0     0   146k      0 --:--:-- --:--:-- --:--:--  146k
"2016-12-18T23:15:03.450Z"
My website’s first CircleCI build was December 18th, 2016 at 11:15pm UTC.