Group posts by year in Jekyll using Liquid

On the list-all-posts page on this blog I wanted to group posts by year and show the year as a section-heading.

I found a StackOverflow answer that suggested using the group_by_exp filter in Liquid. I used it as follows:

{% assign postsByYear = site.posts | group_by_exp:"post", "post.date | date: '%Y'" %}
{% for year in postsByYear %}
<h2>{{ year.name }}</h2>

<div class="post-list">
  {% for post in year.items %}
  <p>
    <a href="{{ post.url }}" class="post-title">{{ post.title }}</a>
    <br>
    <span class="post-description">{{ post.summary }}</span>
  </p>
  {% endfor %}
</div>
{% endfor %}

If you want to further do sub-sections by month then you can use something like this:

{% assign postsByYear = site.posts | group_by_exp:"post", "post.date | date: '%Y'" %}
{% for year in postsByYear %}
<h2>{{ year.name }}</h2>

<div class="month-list">
  {% assign postsByMonth = year.items | group_by_exp:"post", "post.date | date: '%B'" %}
  {% for month in postsByMonth %}
  <h3>{{ month.name }}</h3>

  <div class="post-list">
    {% for post in month.items %}
    <p>
      <a href="{{ post.url }}" class="post-title">{{ post.title }}</a>
      <br>
      <span class="post-description">{{ post.summary }}</span>
    </p>
    {% endfor %}
  </div>
  {% endfor %}
</div>
{% endfor %}