fetchFromGitHub is preferred (as it fetches less):

fetchFromGitHub {
  owner = "stark";
  repo = "siji";
  rev = "95369afac3e661cb6d3329ade5219992c88688c1";
  sha256 = "...";
}

I believe fetchFromGitHub only downloads less when there's a release available, which is not the case here:

  fetchFromGitHub = {
    owner, repo, rev, name ? "${repo}-${rev}-src",
    fetchSubmodules ? false,
    ... # For hash agility
  }@args:
  let
    baseUrl = "https://github.com/${owner}/${repo}";
    passthruAttrs = removeAttrs args [ "owner" "repo" "rev" "fetchSubmodules" ];
  in if fetchSubmodules then
    fetchgit ({
      inherit name rev fetchSubmodules;
      url = "${baseUrl}.git";
    } // passthruAttrs)
  else
    # We prefer fetchzip in cases we don't need submodules as the hash
    # is more stable in that case.
    fetchzip ({
      inherit name;
      url = "${baseUrl}/archive/${rev}.tar.gz";
      meta.homepage = "${baseUrl}/";
    } // passthruAttrs) // { inherit rev; };

There is no release-specific code. For any revision of siji, you can download a tarball using the link https://github.com/stark/siji/archive/<rev>.tar.gz.

Try:

rasen@ashmalko ~ % nix-prefetch-url https://github.com/stark/siji/archive/95369afac3e661cb6d3329ade5219992c88688c1.tar.gz                                                          
downloading ‘https://github.com/stark/siji/archive/95369afac3e661cb6d3329ade5219992c88688c1.tar.gz’... [0/0 KiB, 0.0 Ki [0/0 KiB, 0.0 KiB/s]
path is ‘/nix/store/1lmsk8y7mvlcvxq6rxxz2pnx6jjg01ad-95369afac3e661cb6d3329ade5219992c88688c1.tar.gz’
1pz73pl2ylbhdbikh2nrl39xfyc8am71rhqpzx8lr28slz025q8m

There is also another benefit of fetchFromGitHub. If you examine the code of fetchgit, it does not fetch the full repo, but only N last commits. So if the repo receives too many new commits, fetchgit will fail to checkout to the specified commit as it won't be fetched. (There is a workaround, though.)

You're right, I had a typo in the author's name and I jumped to conclusions. Fixing!

The commit was done on May 13, so better change to 2016-05-13.

I used the date returned by nix-prefetch-git:

❯ nix-prefetch-git https://github.com/stark/siji
Initialized empty Git repository in /tmp/git-checkout-tmp-UuH7HSt2/siji/.git/
remote: Counting objects: 11, done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 11 (delta 0), reused 6 (delta 0), pack-reused 0
Unpacking objects: 100% (11/11), done.
From https://github.com/stark/siji
 * branch            HEAD       -> FETCH_HEAD
Switched to a new branch 'fetchgit'
removing `.git'...

git revision is 95369afac3e661cb6d3329ade5219992c88688c1
path is /nix/store/qv0p80c5pg53qpsvdjbnzk4sdgqf8gdi-siji
git human-readable version is -- none --
Commit date is 2016-05-14 00:30:21 +0530
hash is 1408g4nxwdd682vjqpmgv0cp0bfnzzzwls62cjs9zrds16xa9dpf
{
  "url": "https://github.com/stark/siji",
  "rev": "95369afac3e661cb6d3329ade5219992c88688c1",
  "date": "2016-05-14T00:30:21+05:30",
  "sha256": "1408g4nxwdd682vjqpmgv0cp0bfnzzzwls62cjs9zrds16xa9dpf",
  "fetchSubmodules": true
}

But you're right that the commit is from the 13th... Any idea why?

timezones

Ok, I think 14 is fine, as that's a day the commit was made (in the author's timezone)