diff --git a/src/atom.ml b/src/atom.ml
index f3f6aba..7534ae1 100644
--- a/src/atom.ml
+++ b/src/atom.ml
@@ -1,4 +1,5 @@
 open Post
+(* See https://validator.w3.org/feed/ *)
 
 let format_entry ~title ~date ~url ~contents ~author =
   String.concat ""
@@ -11,7 +12,7 @@ let format_entry ~title ~date ~url ~contents ~author =
   <author>
    <name>|}; author; {|</name>
   </author>
-  <updated>|}; ISO8601.Permissive.string_of_datetime date;{|</updated>
+  <updated>|}; ISO8601.Permissive.string_of_datetimezone (date, 0.) ;{|</updated>
   <link href="|}; url ;{|" rel="alternate" />
   <content type="xhtml">  <div xmlns="http://www.w3.org/1999/xhtml">|};
     contents;
@@ -27,7 +28,7 @@ let format_feed ~title ~subtitle ~author ~feed_url ~site_url ~last_updated ~(ent
   <title>|};  title; {|</title>
   <id>|}; feed_url; {|</id>
   <subtitle>|}; subtitle; {|</subtitle>
-  <updated>|}; ISO8601.Permissive.string_of_datetime last_updated; {|</updated>
+  <updated>|}; ISO8601.Permissive.string_of_datetimezone (last_updated, 0.); {|</updated>
   <link href="|}; feed_url; {|" rel="self"/>
   <link href="|}; site_url; {|"/>
 |}] in