Jekyll Git Updated is a ruby gem for Jekyll static site generator.
This automatically adds a git commit timestamp of the post/page to the git-updated frontmatter key.

Add this line to your Gemfile:

group :jekyll_plugins do
  gem "jekyll-git-updated"
end

And then execute:

$ bundle

Alternatively install the gem yourself as:

$ gem install jekyll-git-updated

and put this in your _config.yml

plugins:
  - jekyll-git-updated

Simply reference the frontmatter in your page/layout.
I.E: {{ page.git-updated }}