try adding last modified date

Gemfile

@@ -3,3 +3,4 @@ source 'https://rubygems.org'
 gem "github-pages"
 gem "just-the-docs"
 gem "jekyll-readme-index"
+gem "jekyll-last-modified-at"

_config.yml

@@ -7,4 +7,7 @@ gh_edit_link: true # show or hide edit this page link
 gh_edit_link_text: "Edit this page on GitHub."
 gh_edit_repository: "https://github.com/12urenloop/docs.12urenloop" # the github URL for your repo
 gh_edit_branch: "master" # the branch that your docs is served from
-gh_edit_view_mode: "tree" # "tree" or "edit" if you want the user to jump into the editor immediately
+gh_edit_view_mode: "tree" # "tree" or "edit" if you want the user to jump into the editor immediately
+
+plugins:
+  - jekyll-last-modified-at

_includes/header_custom.html

@@ -0,0 +1 @@
+{% last_modified_at %}