Add a article:modified_time property for articles (#515)

Merge pull request 515

Author: CookiePLMonster

docs/advanced-usage.md

@@ -122,7 +122,7 @@ The following options can be set for any particular page. While the default opti
   * `type` - The type of things that the page represents. This must be a [Schema.org type](https://schema.org/docs/schemas.html), and will probably usually be something like [`BlogPosting`](https://schema.org/BlogPosting), [`NewsArticle`](https://schema.org/NewsArticle), [`Person`](https://schema.org/Person), [`Organization`](https://schema.org/Organization), etc.
   * `links` - An array of other URLs that represent the same thing that this page represents. For instance, Jane's bio page might include links to Jane's GitHub and Twitter profiles.
   * `date_modified` - Manually specify the `dateModified` field in the JSON-LD output to override Jekyll's own `dateModified`.
-  This field will take **first priority** for the `dateModified` JSON-LD output. This is useful when the file timestamp does not match the true time that the content was modified. A user may also install [Last Modified At](https://github.com/gjtorikian/jekyll-last-modified-at) which will offer an alternative way of providing for the `dateModified` field.
+  This field will take **first priority** for the `dateModified` JSON-LD output and the article modification time for Open Graph (`article:modified_time`). This is useful when the file timestamp does not match the true time that the content was modified. A user may also install [Last Modified At](https://github.com/gjtorikian/jekyll-last-modified-at) which will offer an alternative way of providing for the `dateModified` field.
 
 ### Customizing image output
 

lib/template.html

@@ -42,9 +42,12 @@
   {% endif %}
 {% endif %}
 
-{% if page.date %}
+{% if seo_tag.date_published %}
   <meta property="og:type" content="article" />
-  <meta property="article:published_time" content="{{ page.date | date_to_xmlschema }}" />
+  <meta property="article:published_time" content="{{ seo_tag.date_published | date_to_xmlschema }}" />
+  {% if seo_tag.date_modified %}
+    <meta property="article:modified_time" content="{{ seo_tag.date_modified | date_to_xmlschema }}" />
+  {% endif %}
 {% else %}
   <meta property="og:type" content="website" />
 {% endif %}