Drupal - How to print date in format «long» on node.html.twig template?

you are not using the actual timestamp. Here's what you should do:

{% set date = node.createdtime|format_date('long') %}

See? You only need to replace created with createdtime. Hope this helps.

Try in this way.

{% set date = node.created %}
{{ set date|date("m/d/Y") }}

• Refer here to know more about filter date in TWIG.
• Refer here to know more about date formats in php.

Hope this will help you.