Where is the node created timestamp stored in the database?
The created date is stored in the node_field_data table as a UNIX timestamp in a cloumn called created.

Here's the code you would use to fetch the nodes that has been created X days ago, using a direct select query.

$day = 86400; // One day in seconds.
$days = 3; // The amount of days ago to fetch from.

$nids = \Drupal::database()-&gt;select('node_field_data', 'n')
  -&gt;addField('n', 'nid')
  -&gt;condition('n.created', \Drupal::time()-&gt;getCurrentTime() - $days * $day, '&gt;')
  -&gt;execute()
  -&gt;fetchField();

$nodes = \Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;loadMultiple($nids);


Make sure to inject the database and the entity_type.manager services (if possible) when implementing the code yourself.