Updated Jekyll archive template. Reduced code duplication and heading bug fix.

archive.html

<section id="archive">
  {% capture this_year %}{{ site.time | date: '%Y' }}{% endcapture %}
  {% capture post_year %}{{ site.posts[0].date | date: '%Y' }}{% endcapture %}

  {% if this_year == post_year %}
  <h3>This year's posts</h3>
  {% else %}
  <h3>{{ post_year }}</h3>
  {% endif %}

  {%for post in site.posts %}
    {% unless post.next %}
      <ul class="this">
    {% else %}
      {% capture year %}{{ post.date | date: '%Y' }}{% endcapture %}
      {% capture nyear %}{{ post.next.date | date: '%Y' }}{% endcapture %}
      {% if year != nyear %}
        </ul>
        <h3>{{ post.date | date: '%Y' }}</h3>
        <ul class="past">
      {% endif %}
    {% endunless %}
      <li><time>{{ post.date | date:"%d %b" }}</time><a href="{{ post.url }}">{{ post.title }}</a></li>
  {% endfor %}
  </ul>
</section>