datetime must be valid date format, see: http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#concept-datetime

.themes/classic/source/_includes/archive_post.html

@@ -1,6 +1,6 @@
 {% capture category %}{{ post.categories | size }}{% endcapture %}
 <h1><a href="{{ root_url }}{{ post.url }}">{{post.title}}</a></h1>
-<time datetime="{{ post.date | datetime }}" pubdate>{{ post.date | date: "<span class='month'>%b</span> <span class='day'>%d</span> <span class='year'>%Y</span>"}}</time>
+<time datetime="{{ post.date | datetime | date_to_xmlschema }}" pubdate>{{ post.date | date: "<span class='month'>%b</span> <span class='day'>%d</span> <span class='year'>%Y</span>"}}</time>
 {% if category != '0' %}
 <footer>
   <span class="categories">posted in {{ post.categories | category_links }}</span>

.themes/classic/source/_includes/post/date.html

@@ -3,8 +3,8 @@
 {% capture updated %}{{ page.updated }}{{ post.updated }}{% endcapture %}
 {% capture was_updated %}{{ updated | size }}{% endcapture %}
 {% if has_date != '0' %}
-<time datetime="{{ date | datetime }}" pubdate {% if updated %} updated {% endif %}>{{ date | ordinalize }}</time>
+<time datetime="{{ date | datetime | date_to_xmlschema }}" pubdate {% if updated %} updated {% endif %}>{{ date | ordinalize }}</time>
 {% endif %}
 {% if was_updated != '0' %}
-<time class="updated" datetime="{{ updated | datetime }}">{{ updated | ordinalize }}</time>
+<time class="updated" datetime="{{ updated | datetime | date_to_xmlschema }}">{{ updated | ordinalize }}</time>
 {% endif %}