{% 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 %}