{% if include.params.filter %}
  {% assign filter = include.params.filter %}
{% endif %}

{% assign split_mark = '<|>' %}

{% assign dates = '' %}
{% for post in site.posts %}
  {% assign name = post.date | date: filter %}
  {% assign dates = dates | append: split_mark  | append: name %}
{% endfor %}

{% assign return = dates
  | remove_first: split_mark
  | split: split_mark
  | sort: self
  | uniq %}