{% 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 %}