Code snippet that can be used to to get year and month from date_iso8601 for computed field in drupal 8.
Example with date()
$date = $entity->created->value;
$date = date('Y-m-d H:i:s', $date);
Example get year
$date = $node->getCreatedTime();
$year = date('Y', $date);
Example to get year with date.formatter service:
$date = $node->getCreatedTime();
$year = \Drupal::service('date.formatter')->format($date, 'html_year');
Example to get year with DrupalDateTime class:
$date = $node->getCreatedTime();
$date = DrupalDateTime::createFromTimestamp($date);
$year = $date->format('Y');
