Automatically updating the created date for talk nodes, to match the date of the latest event for that talk.

custom.module

<?php

/**
 * @file
 * Custom module.
 */

declare(strict_types=1);

use Drupal\custom\Entity\Node;

/**
 * Implements hook_entity_type_build().
 */
function custom_entity_type_build(array &$entityTypes): void {
  if (isset($entityTypes['node'])) {
    $entityTypes['node']->setClass(Node::class);
  }
}

custom.services.yml

services:
  Drupal\custom\EventSubscriber\UpdateTalkCreatedDateOnSave:
    tags:
      - { name: event_subscriber }

Node.php

<?php

declare(strict_types=1);

namespace Drupal\custom\Entity;

use Drupal\node\Entity\Node as CoreNode;
use Drupal\paragraphs\ParagraphInterface;
use Illuminate\Support\Collection;

final class Node extends CoreNode {

  /**
   * Find the date for the latest event.
   *
   * @return string|null
   */
  public function findLatestEventDate(): ?string {
    if (!$this->hasField('field_events')) {
      return NULL;
    }

    return Collection::make($this->get('field_events')->referencedEntities())
      ->map(fn(ParagraphInterface $event) => $event->get('field_date')
        ->getString())
      ->max();
  }

}

UpdatesTalkCreatedDateTest.php

<?php

declare(strict_types=1);

namespace Drupal\Tests\custom\Kernel;

use Carbon\Carbon;
use Drupal\Core\Entity\EntityInterface;
use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
use Drupal\node\Entity\Node;
use Drupal\paragraphs\Entity\Paragraph;
use Drupal\paragraphs\ParagraphInterface;

final class UpdatesTalkCreatedDateTest extends EntityKernelTestBase {

  /**
   * {@inheritDoc}
   */
  public static $modules = [
    // Core.
    'node',
    'file',
    'datetime',

    // Contrib.
    'entity_reference_revisions',
    'paragraphs',
    'hook_event_dispatcher',

    // Custom.
    'custom',
    'custom_test',
  ];

  public function testCreatingNode() {
    $eventDate = Carbon::today()->addWeek();
    $eventDateToFormat = $eventDate->format(DateTimeItemInterface::DATE_STORAGE_FORMAT);
    $eventDateToTimestamp = $eventDate->getTimestamp();

    $talk = $this->createTalk($eventDateToFormat);

    $this->assertEqual($eventDateToTimestamp, $talk->get('created')
      ->getString());
  }

  public function testUpdatingNode() {
    $talk = $this->createTalk();

    $eventDate = Carbon::today()->addWeek();
    $eventDateToFormat = $eventDate->format(DateTimeItemInterface::DATE_STORAGE_FORMAT);
    $eventDateToTimestamp = $eventDate->getTimestamp();

    $event = $this->createEvent($eventDateToFormat);
    $talk->set('field_events', [$event]);
    $talk->save();

    $this->assertEqual($eventDateToTimestamp, $talk->get('created')
      ->getString());
  }

  protected function setUp() {
    parent::setUp();

    $this->installEntitySchema('paragraph');
    $this->installSchema('node', ['node_access']);

    $this->installConfig(['custom_test']);
  }

  private function createTalk(?string $eventDateToFormat = NULL): EntityInterface {
    if ($eventDateToFormat) {
      $event = $this->createEvent($eventDateToFormat);
    }

    $talk = Node::create([
      'title' => 'TDD - Test Driven Drupal',
      'type' => 'talk',
    ]);

    if (isset($event)) {
      $talk->set('field_events', [$event]);
    }

    $talk->save();

    return $talk;
  }

  private function createEvent(string $eventDateToFormat): ParagraphInterface {
    $event = Paragraph::create([
      'field_date' => $eventDateToFormat,
      'type' => 'event',
    ]);

    $event->save();

    return $event;
  }

}

UpdateTalkCreatedDateOnSave.php

<?php

declare(strict_types=1);

namespace Drupal\custom\EventSubscriber;

use Carbon\Carbon;
use Drupal\Core\Entity\EntityInterface;
use Drupal\hook_event_dispatcher\Event\Entity\BaseEntityEvent;
use Drupal\hook_event_dispatcher\HookEventDispatcherInterface;
use Drupal\paragraphs\ParagraphInterface;
use Illuminate\Support\Collection;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Set the created date for a talk to be the last date that the talk is given.
 */
final class UpdateTalkCreatedDateOnSave implements EventSubscriberInterface {

  public static function getSubscribedEvents() {
    return [
      HookEventDispatcherInterface::ENTITY_INSERT => 'entityInsertOrUpdate',
      HookEventDispatcherInterface::ENTITY_UPDATE => 'entityInsertOrUpdate',
    ];
  }

  public function entityInsertOrUpdate(BaseEntityEvent $event): void {
    if ($event->getEntity()->getEntityTypeId() != 'node') {
      return;
    }

    if ($event->getEntity()->bundle() != 'talk') {
      return;
    }

    $this->updateCreatedDate($event->getEntity());
  }

  private function updateCreatedDate(EntityInterface $talk): void {
    /** @var \Drupal\custom\Entity\Node $talk */
    if (!$eventDate = $talk->findLatestEventDate()) {
      return;
    }

    $talkDate = Carbon::parse($eventDate)->getTimestamp();

    if ($talkDate == $talk->get('created')->getString()) {
      return;
    }

    $talk->set('created', $talkDate);
  }

  /**
   * Find the date for the latest event.
   *
   * @return string|null
   */
  private function findLatestEventDate(EntityInterface $talk) {
    return Collection::make($talk->get('field_events')->referencedEntities())
      ->map(fn(ParagraphInterface $event) => $event->get('field_date')
        ->getString())
      ->max();
  }

}