Add published date metadata to posts.

source/pages/templates/post.html

@@ -3,6 +3,10 @@
 
 {% extends "base.html" %}
 
+{% block head %}
+  <meta property="og:article:published_time" content="{{ publishedDate.toISOString() }}" />
+{% endblock %}
+
 {% block body %}
   <main id="main">
     <section id="post">

source/scripts/html.ts

@@ -136,6 +136,7 @@ async function entry(): Promise<void> {
             year: Number(year)
           },
           pluralize,
+          publishedDate: new Date(Number(year), monthNumber, 0, 12, 0, 0),
           statistic
         }
       );