diff --git a/src/indices.lisp b/src/indices.lisp
index 29a1e4a..65732cd 100644
--- a/src/indices.lisp
+++ b/src/indices.lisp
@@ -17,7 +17,11 @@
 (defclass date-index (index) ())
 (defclass int-index (index) ())
 (defclass url-index (index)
-  ((urls :initform nil :initarg :urls :accessor urls)))
+  ((urls :initform nil :initarg :urls :accessor urls)
+   (pubdate :initform (local-time:format-rfc3339-timestring nil
+                                                            (local-time:now))
+            :initarg :pubdate
+            :accessor index-pubdate)))
 
 (defmethod page-url ((object index))
   (index-id object))
diff --git a/themes/sitemap.tmpl b/themes/sitemap.tmpl
index d9b5f91..4833858 100644
--- a/themes/sitemap.tmpl
+++ b/themes/sitemap.tmpl
@@ -6,6 +6,7 @@
     {foreach $url in $content.urls}
     <url>
         <loc>{$config.domain}/{$url}</loc>
+        <lastmod>{$content.pubdate}</lastmod>
     </url>
     {/foreach}
 </urlset>