make date_format a site-wide variable

_config.yml

@@ -14,3 +14,5 @@ plugins:
  - jekyll-feed
  - jekyll-seo-tag
  - jekyll-paginate
+
+date_format: "%b %-d, %Y"

_layouts/home.html

@@ -11,7 +11,10 @@ layout: default
         {{ post.title | escape }}
       </a>
     </h2>
-    <span>{{ post.date | date: "%b %-d, %Y" }}</span>
+    <span>{{ post.date | date: site.date_format }}</span>
+    {%- if post.updated %}
+    <span>(Updated {{ post.updated | date: site.date_format }})</span>
+    {% endif -%}
     <p>{{ post.description }}</p>
   </li>
   {%- endfor -%}

_layouts/post.html

@@ -13,12 +13,12 @@ layout: default
       </span>
       <!-- • -->
       <time datetime="{{ page.date | date_to_xmlschema }}" itemprop="datePublished">
-        {{ page.date | date: "%b %-d, %Y" }}
+        {{ page.date | date: site.date_format }}
       </time>
       •
       {%- if page.updated %}
       <time datetime="{{ page.updated | date_to_xmlschema }}" itemprop="dateModified">
-        (Updated {{ page.updated | date: "%b %-d, %Y" }})
+        (Updated {{ page.updated | date: site.date_format }})
       </time>
       •
       {% endif -%}