diff --git a/_plugins/add-posts-update-date.rb b/_plugins/add-posts-update-date.rb
new file mode 100644
index 0000000..099ebbc
--- /dev/null
+++ b/_plugins/add-posts-update-date.rb
@@ -0,0 +1,4 @@
+Jekyll::Hooks.register :posts, :pre_render do |post|
+  mtime = File.mtime post.path
+  post.data['update_date'] = mtime unless post.data['date'].to_date == mtime.to_date
+end