Improve the way to define and use date-format.

_includes/related-posts.html

@@ -53,7 +53,7 @@
         <a href="{{ post.url | relative_url }}">
           <div class="card-body">
             <span class="timeago small">
-                {{ post.date | date: POST_DATE }}
+                {{ post.date | date: site.data.date_format.post }}
                 <i class="unloaded">{{ post.date | date_to_xmlschema }}</i>
             </span>
             <h3 class="pt-0 mt-1 mb-3" data-toc-skip>{{ post.title }}</h3>