diff --git a/lib/volumen/server.rb b/lib/volumen/server.rb
index 43fed7c..31c0968 100644
--- a/lib/volumen/server.rb
+++ b/lib/volumen/server.rb
@@ -324,7 +324,9 @@ module Volumen
 
     def sitemap_xml
       urls = published_posts.map do |post|
-        "  <url><loc>#{escape("#{base_url}/#{post.slug}")}</loc></url>"
+        url = "  <url><loc>#{escape("#{base_url}/#{post.slug}")}</loc>"
+        url << "<lastmod>#{escape(post.date_string)}</lastmod>" if post.date_string
+        url << "</url>"
       end.join("\n")
       <<~XML
         <?xml version="1.0" encoding="UTF-8"?>