Datetime format (mmistakes#2844)

* datetime_format

* page__meta

* page__date

* page__date test

* update docs

* update docs

Author: luweizheng

_includes/page__date.html

@@ -1,5 +1,6 @@
+{% assign date_format = site.date_format | default: "%B %-d, %Y" %}
 {% if page.last_modified_at %}
-  <p class="page__date"><strong><i class="fas fa-fw fa-calendar-alt" aria-hidden="true"></i> {{ site.data.ui-text[site.locale].date_label | default: "Updated:" }}</strong> <time datetime="{{ page.last_modified_at | date: "%Y-%m-%d" }}">{{ page.last_modified_at | date: "%B %-d, %Y" }}</time></p>
+  <p class="page__date"><strong><i class="fas fa-fw fa-calendar-alt" aria-hidden="true"></i> {{ site.data.ui-text[site.locale].date_label | default: "Updated:" }}</strong> <time datetime="{{ page.last_modified_at | date: "%Y-%m-%d" }}">{{ page.last_modified_at | date: date_format }}</time></p>
 {% elsif page.date %}
-  <p class="page__date"><strong><i class="fas fa-fw fa-calendar-alt" aria-hidden="true"></i> {{ site.data.ui-text[site.locale].date_label | default: "Updated:" }}</strong> <time datetime="{{ page.date | date_to_xmlschema }}">{{ page.date | date: "%B %-d, %Y" }}</time></p>
+  <p class="page__date"><strong><i class="fas fa-fw fa-calendar-alt" aria-hidden="true"></i> {{ site.data.ui-text[site.locale].date_label | default: "Updated:" }}</strong> <time datetime="{{ page.date | date_to_xmlschema }}">{{ page.date | date: date_format }}</time></p>
 {% endif %}

_includes/page__meta.html

@@ -5,7 +5,8 @@
       {% assign date = document.date %}
       <span class="page__meta-date">
         <i class="far {% if include.type == 'grid' and document.read_time and document.show_date %}fa-fw {% endif %}fa-calendar-alt" aria-hidden="true"></i>
-        <time datetime="{{ date | date_to_xmlschema }}">{{ date | date: "%B %-d, %Y" }}</time>
+        {% assign date_format = site.date_format | default: "%B %-d, %Y" %}
+        <time datetime="{{ date | date_to_xmlschema }}">{{ date | date: date_format }}</time>
       </span>
     {% endif %}
 

docs/_docs/05-configuration.md

@@ -285,6 +285,12 @@ defaults:
 
 To disable post date for a post, add `show_date: false` to its YAML Front Matter, overriding what was set in `_config.yml`.
 
+When dates are shown on blog posts or pages, a date format will be chosen to format the date string. The default format is `"%B %-d, %Y"`, which will be displayed like "February 24, 2016". You can choose your date format by referencing this [cheat sheet](https://www.shortcutfoo.com/app/dojos/ruby-date-format-strftime/cheatsheet). For example, use your date format in `_config.yml`.
+
+```yaml
+date_format: "%Y-%m-%d"
+```
+
 ### Reading time
 
 Enable estimated reading time snippets with `read_time: true` in YAML Front Matter. `200` has been set as the default words per minute value --- which can be changed by adjusting `words_per_minute:` in `_config.yml`.