for simiki

atom.xml

<?xml version="1.0" encoding="utf-8"?>
{% if site.url %}
  {% set site_url = "%s%s"|format(site.url, site.root) %}
{% else %}
  {% set site_url = site.url %}
{% endif %}
<feed xmlns="http://www.w3.org/2005/Atom">
  <generator uri="http://simiki.org/" version="{{ site.version }}">Simiki</generator>
  <title>{{ site.title }}</title>
  <link href="{{ site_url }}/" />
  <link href="{{ site_url }}/atom.xml" rel="self" type="application/atom+xml" />
  <updated>{{ site.time|rfc3339 }}</updated>
  <id>{{ site_url }}</id>
  <author>
    <name>{{ site.author|e }}</name>
  </author>
  {% for f, page in pages.iteritems() %}
    {% set uri = site_url ~ f|replace('content/', '', 1)|replace('.md', '.html', 1) %}
  <entry>
    <id>{{ uri }}</id>
    <title>{{ page.title|e }}</title>
    <link href="{{ uri }}" />
    <published>{{ page.date|rfc3339 }}</published>

    {% if page.updated %}
    <updated>{{ page.updated|rfc3339 }}</updated>
    {% else %}
    <updated>{{ page.date|rfc3339 }}</updated>
    {% endif %}

    {% if page.category %}
    <category term="{{ page.category }}" />
    {% endif %}

    {% if page.content %}
    <content type="html">{{ page.content|e }}</content>
    {% endif %}
    {% if page.summary %}
    <summary>{{ page.summary|e }}</summary>
    {% endif %}
  </entry>
  {% endfor %}
</feed>

updated must exists, from rfc4287:

atom:entry elements MUST contain exactly one atom:updated element