diff --git a/lib/rdoc/generator/template/rails/_head.rhtml b/lib/rdoc/generator/template/rails/_head.rhtml
index 09963076..bb0449e6 100644
--- a/lib/rdoc/generator/template/rails/_head.rhtml
+++ b/lib/rdoc/generator/template/rails/_head.rhtml
@@ -20,3 +20,4 @@
 <meta property="og:image" content="<%= canonical_url("/i/logo.svg") %>">
 <% end %>
 <meta property="og:type" content="article">
+<meta property="article:modified_time" content="<%= og_modified_time %>">
diff --git a/lib/sdoc/helpers.rb b/lib/sdoc/helpers.rb
index e92a3934..7e0ffd88 100644
--- a/lib/sdoc/helpers.rb
+++ b/lib/sdoc/helpers.rb
@@ -1,6 +1,6 @@
 module SDoc::Helpers
-  require_relative "helpers/github"
-  include SDoc::Helpers::GitHub
+  require_relative "helpers/git"
+  include SDoc::Helpers::Git
 
   def link_to(text, url, html_attributes = {})
     return h(text) if url.nil?
@@ -50,6 +50,10 @@ def og_title(title)
     "#{h title}#{" (#{project})" unless project.empty?}"
   end
 
+  def og_modified_time
+    git_head_timestamp if git?
+  end
+
   def page_description(leading_html, max_length: 160)
     return if leading_html.nil? || !leading_html.include?("</p>")
 
diff --git a/lib/sdoc/helpers/github.rb b/lib/sdoc/helpers/git.rb
similarity index 81%
rename from lib/sdoc/helpers/github.rb
rename to lib/sdoc/helpers/git.rb
index a69508ad..def2016d 100644
--- a/lib/sdoc/helpers/github.rb
+++ b/lib/sdoc/helpers/git.rb
@@ -1,4 +1,4 @@
-module SDoc::Helpers::GitHub
+module SDoc::Helpers::Git
   def github_url(relative_path, line: nil)
     return unless github?
     line = "#L#{line}" if line
@@ -24,6 +24,14 @@ def git_head_sha1
     end
   end
 
+  attr_writer :git_head_timestamp
+
+  def git_head_timestamp
+    @git_head_timestamp ||= Dir.chdir(@options.root) do
+      `git show -s --format=%cI HEAD` .chomp
+    end
+  end
+
   attr_writer :git_origin_url
 
   def git_origin_url
diff --git a/spec/helpers_spec.rb b/spec/helpers_spec.rb
index b3c8f9ce..96ff2173 100644
--- a/spec/helpers_spec.rb
+++ b/spec/helpers_spec.rb
@@ -270,6 +270,26 @@ module Foo; module Bar; module Qux; end; end; end
     end
   end
 
+  describe "#og_modified_time" do
+    it "returns the commit time of the most recent commit in HEAD" do
+      @helpers.git_bin_path = "path/to/git"
+      @helpers.git_head_timestamp = "1999-12-31T12:34:56Z"
+
+      _(@helpers.og_modified_time).must_equal "1999-12-31T12:34:56Z"
+    end
+
+    it "returns the commit time of the most recent commit in HEAD (smoke test)" do
+      _(@helpers.og_modified_time).
+        must_match %r"\A\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[-+]\d{2}:\d{2}\z"
+    end
+
+    it "returns nil when git is not installed" do
+      @helpers.git_bin_path = ""
+
+      _(@helpers.og_modified_time).must_be_nil
+    end
+  end
+
   describe "#page_description" do
     it "extracts the description from the leading paragraph" do
       _(@helpers.page_description(<<~HTML)).must_equal "leading"