Improve the way to define and use date-format.

_includes/date-format.html

@@ -1,10 +0,0 @@
-<!--
- Define the liquid date formats.
- v2.0
- https://github.com/cotes2020/jekyll-theme-chirpy
- © 2019 Cotes Chung
- Published under the MIT License
--->
-
-{% assign TOOLTIP_DATE = "%a, %b %e, %Y, %l:%M %p %z" %}
-{% assign POST_DATE = "%b %e, %Y" %}

_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>