Change article date format
So after spending more time than I care to admit looking into this, I've come up with two solutions to this.

Solution 1

One way to do it is with the date function that darol100 mentioned in his answer. Modifying his example for my case, in my node--article.html.twig file, I changed:

{% trans %}Submitted by {{ author_name }} on {{ date }}{% endtrans %}


to

{% set createdDate = node.getCreatedTime|date('j F Y') %}
{% trans %}Submitted by {{ author_name }} on {{ createdDate }} {% endtrans %}


The advantage to this approach is that it's easy and quick. The disadvantage is I'm not using Drupal's built-in date format system.

Solution 2

In order to use Drupal's date format system, I first created my custom format at admin/config/regional/date-time. Then I edited node--article.html.twig as follows:

{% set createdDate = node.getCreatedTime|format_date('my_custom_format') %}
{% trans %}Submitted by {{ author_name }} on {{ createdDate }} {% endtrans %}


This assumes I named my custom format "My Custom Format", which results in the machine name my_custom_format.

While this solution requires an extra step, I feel it's the more Drupal way to do it.

I learned about it on this page about Drupal's Twig Filters.