diff --git a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeFieldItemList.php b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeFieldItemList.php
index e102c0e..22c6131 100644
--- a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeFieldItemList.php
+++ b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeFieldItemList.php
@@ -24,26 +24,33 @@ class DateTimeFieldItemList extends ConfigFieldItemList {
    * {@inheritdoc}
    */
   public function defaultValuesForm(array &$form, array &$form_state) {
-    $default_value = parent::getDefaultValue();
+    if (empty($this->getFieldDefinition()->default_value_function)) {
+      $default_value = $this->getFieldDefinition()->default_value;
 
-    $element = array(
-      '#parents' => array('default_value_input'),
-      'default_date' => array(
-        '#type' => 'select',
-        '#title' => t('Default date'),
-        '#description' => t('Set a default value for this date.'),
-        '#default_value' => isset($default_value[0]['default_date']) ? $default_value[0]['default_date'] : '',
-        '#options' => array(static::DEFAULT_VALUE_NOW => t('The current date')),
-        '#empty_value' => '',
-      )
-    );
+      $element = array(
+        '#parents' => array('default_value_input'),
+        'default_date' => array(
+          '#type' => 'select',
+          '#title' => t('Default date'),
+          '#description' => t('Set a default value for this date.'),
+          '#default_value' => isset($default_value[0]['default_date']) ? $default_value[0]['default_date'] : '',
+          '#options' => array(static::DEFAULT_VALUE_NOW => t('The current date')),
+          '#empty_value' => '',
+        )
+      );
 
-    return $element;
+      return $element;
+    }
   }
 
   /**
    * {@inheritdoc}
    */
+  public function defaultValuesFormValidate(array $element, array &$form, array &$form_state) { }
+
+  /**
+   * {@inheritdoc}
+   */
   public function defaultValuesFormSubmit(array $element, array &$form, array &$form_state) {
     if ($form_state['values']['default_value_input']['default_date']) {
       return array($form_state['values']['default_value_input']);
@@ -57,7 +64,7 @@ public function defaultValuesFormSubmit(array $element, array &$form, array &$fo
   public function getDefaultValue() {
     $default_value = parent::getDefaultValue();
 
-    if ($default_value && $default_value[0]['default_date'] == static::DEFAULT_VALUE_NOW) {
+    if ($default_value && isset($default_value[0]['default_date']) && $default_value[0]['default_date'] == static::DEFAULT_VALUE_NOW) {
       // A default value should be in the format and timezone used for date
       // storage.
       $date = new DrupalDateTime('now', DATETIME_STORAGE_TIMEZONE);
diff --git a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeItem.php b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeItem.php
index df64d25..1918409 100644
--- a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeItem.php
+++ b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldType/DateTimeItem.php
@@ -30,12 +30,12 @@
 class DateTimeItem extends ConfigFieldItemBase implements PrepareCacheInterface {
 
   /**
-   * Defines the field type as date.
+   * Value for the 'datetime_type' setting: store only a date.
    */
   const DATETIME_TYPE_DATE = 'date';
 
   /**
-   * Defines the field type as datetime.
+   * Value for the 'datetime_type' setting: store a date and time.
    */
   const DATETIME_TYPE_DATETIME = 'datetime';