try last-modified-at plugin, see /jardin

Gemfile

@@ -23,5 +23,5 @@ group :jekyll_plugins do
   gem "jekyll-paginate"
   gem "jekyll-sitemap"
   gem "jekyll-include-cache"
-  gem "jekyll-menus"
+  gem "jekyll-last-modified-at"
 end

_config.yml

@@ -218,12 +218,14 @@ plugins:
   - jekyll-paginate
   - jekyll-sitemap
   - jekyll-include-cache
+  - jekyll-last-modified-at
 
 # mimic GitHub Pages with --safe
 whitelist:
   - jekyll-paginate
   - jekyll-sitemap
   - jekyll-include-cache
+  - jekyll-last-modified-at
 
 
 # Archives