diff --git a/source/blog/tag.xml b/source/blog/tag.xml
index ea03c0f4..3620c9e4 100644
--- a/source/blog/tag.xml
+++ b/source/blog/tag.xml
@@ -9,7 +9,7 @@ generator: [posts_tag_index]
     <language>en</language>
     {% if site.description %}<description>{{ site.description|raw }}</description>{% endif %}
     <atom:link href="{{ site.url }}{{ page.url }}" rel="self" type="application/rss+xml" />
-    <lastBuildDate>{{ site.calculated_date | date('c') }}</lastBuildDate>
+    <lastBuildDate>{{ site.calculated_date|date('r') }}</lastBuildDate>
       {% for post in page.tag_posts|slice(0, 10) %}
       <item>
         <title>{{ post.title }}</title>