Add stubs for datetime field type

stubs/Drupal/datetime/Plugin/Field/FieldType/DateTimeItem.stub

@@ -0,0 +1,13 @@
+<?php
+
+namespace Drupal\datetime\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldItemBase;
+
+/**
+ * @property ?string $value
+ * @property-read ?\Drupal\Core\Datetime\DrupalDateTime $date
+ */
+class DateTimeItem extends FieldItemBase implements DateTimeItemInterface {
+
+}

tests/src/Type/data/field-types.php

@@ -18,6 +18,7 @@
 use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
 use Drupal\Core\Field\Plugin\Field\FieldType\UriItem;
 use Drupal\Core\Field\Plugin\Field\FieldType\UuidItem;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItem;
 use Drupal\file\Plugin\Field\FieldType\FileItem;
 use Drupal\file\Plugin\Field\FieldType\FileUriItem;
 use Drupal\link\Plugin\Field\FieldType\LinkItem;
@@ -137,6 +138,13 @@
 assertType(UuidItem::class, $uuid_field);
 assertType('string', $uuid_field->value);
 
+// DateTimeItem.
+$datetime_field = $node->get('field_datetime')->first();
+assert($datetime_field instanceof DateTimeItem);
+assertType(DateTimeItem::class, $datetime_field);
+assertType('string|null', $datetime_field->value);
+assertType('Drupal\Core\Datetime\DrupalDateTime|null', $datetime_field->date);
+
 // FileItem.
 $file_field = $node->get('field_file')->first();
 assert($file_field instanceof FileItem);