Fixed the template of Sitemap and Feed.

feed.xml

@@ -30,8 +30,8 @@ layout: compress
     <title>{{ post.title }}</title>
     <link href="{{ post_absolute_url }}" rel="alternate" type="text/html" title="{{ post.title }}" />
     <published>{{ post.date | date_to_xmlschema }}</published>
-  {% if post.lastmod %}
-    <updated>{{ post.lastmod | date_to_xmlschema }}</updated>
+  {% if post.seo.date_modified %}
+    <updated>{{ post.seo.date_modified | date_to_xmlschema }}</updated>
   {% else %}
     <updated>{{ site.time | date_to_xmlschema }}</updated>
   {% endif %}