Example how to set date field programmatically drupal 8
function mymodule_entity_presave(EntityInterface $entity)
{
if ($entity->getEntityTypeId() == 'node') {
$dateTime = \DateTime::createFromFormat('Y-m-d',date('Y-m-d', time()));
$publicationDate = $dateTime->format('Y-m-d');
$entity->set('field_publication_date', $publicationDate);
}
}
