Time of creation of a Github Fork

If the first commit of your repository was made not long after the repo itself was created, you could always approximate the creation time of the repo by doing git log --reverse and find the authored date of the commit, or if the root commit was never modified, you can pass git log --reverse --format=fuller to get the committed date too.

You could use the GitHub Repos API, it does contain the creation date:

  GET /users/:user/repos

    ...
    "pushed_at": "2011-01-26T19:06:43Z",
    "created_at": "2011-01-26T19:01:12Z",   <====
    "updated_at": "2011-01-26T19:14:43Z"
  }

More precisely, using jq for JSON filtering, to get creation date of my old GitHub repository VonC/b2d:

VonC@vonc MINGW64 /c/Users/vonc/git
$ curl https://api.github.com/users/VonC/repos | jq '.[] | select(.name=="b2d") | .created_at'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  153k  100  153k    0     0   105k      0  0:00:01  0:00:01 --:--:--  105k
"2015-03-28T19:58:35Z"