uber_publisher_daily_cartoons-8.x-6.x-dev/modules/uber_publisher_monthly_caricature/uber_publisher_monthly_caricature.module

<?php

/**
 * @file
 * Contains uber_publisher_monthly_caricature.module.
 */

use Drupal\Core\Entity\EntityInterface;

/**
 * Implements hook_node_presave().
 */
function uber_publisher_monthly_caricature_node_presave(EntityInterface $entity) {
  if ($entity->getType() == 'caricature') {
    if (!empty($entity->get('field_date')->getValue()[0]['value'])) {
      $value = date("Ym", strtotime($entity->get('field_date')->getValue()[0]['value']));
      $entity->set('field_year_month', $value);
    }
  }
}