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function mymodule_node_presave(\Drupal\Core\Entity\EntityInterface $entity) {
  if ($entity->bundle() == 'your_content_type' && !$entity->isNew()) {
    // Check and set the start date.
    if (!empty($entity->get('field_date')->value)) {
      $start_date = new \DateTime($entity->get('field_date')->value, new \DateTimeZone('Europe/Zurich'));
      $start_date->setTimezone(new \DateTimeZone('UTC'));
      $entity->set('field_date', $start_date->format('Y-m-d\TH:i:s'));
    }

    // Check and set the end date.
    if (!empty($entity->get('field_date')->end_value)) {
      $end_date = new \DateTime($entity->get('field_date')->end_value, new \DateTimeZone('Europe/Zurich'));
      $end_date->setTimezone(new \DateTimeZone('UTC'));
      $entity->set('field_date', ['end_value' => $end_date->format('Y-m-d\TH:i:s')]);
    }
  }
}