Add Last Modified Datetime to Meta

Feeling Responsive includes _meta_information.html when show_meta is truthy. Meta information output includes the date with a calendar icon. It would be beneficial for user agents and the Web infrastructure if we also included the last modified time.

Last modified time can be included using the jekyll-last-modified-at gem. Here’s an example update to the meta info include file to incorporate this data when the gem is present (and enabled via config):

{% if page.date %}
  <span class="icon-calendar pr20"> Published: <time datetime="{{ page.date | date_to_xmlschema }}" itemprop="datePublished"> {{ page.date | date: "%Y-%m-%d" }}</time></span>
{% endif %}
{% if page.last_modified_at %}
  <span class="icon-calendar pr20"> Updated: <time datetime="{{ page.last_modified_at | date_to_xmlschema }}" itemprop="datePublished"> {{ page.last_modified_at | date: "%Y-%m-%d" }}</time></span>
{% endif %}

The above looks like this when output:

Considerations:

This is a little verbose, mind you, as user agents and readers alike may benefit from conditionally visually displaying one or the other depending on which is available and more recent. In other words, only show one calendar icon and date, specify which it is (one of Published or Last Modified), but output both, if available, to the page (one as meta with microformat data, or using JSON-LD).

[ghost]

https://github.com/jekyll/jekyll-seo-tag/pull/151/files#r103643150

[ghost]

I jumped the gun on closing this. I believe the gem may still see GH pages support once some validation occurs.