diff -u b/core/modules/datetime/src/Plugin/migrate/field/d6/DateField.php b/core/modules/datetime/src/Plugin/migrate/field/d6/DateField.php
--- b/core/modules/datetime/src/Plugin/migrate/field/d6/DateField.php
+++ b/core/modules/datetime/src/Plugin/migrate/field/d6/DateField.php
@@ -3,7 +3,7 @@
 namespace Drupal\datetime\Plugin\migrate\field\d6;
 
 use Drupal\migrate\Plugin\MigrationInterface;
-use Drupal\migrate\Row;
+use Drupal\migrate\MigrateException;
 use Drupal\migrate_drupal\Plugin\migrate\field\FieldPluginBase;
 
 /**
@@ -43,7 +43,6 @@
    * {@inheritdoc}
    */
   public function processFieldValues(MigrationInterface $migration, $field_name, $data) {
-
     switch($data['type']) {
       case 'date':
         $from_format = 'Y-m-d\TH:i:s';
@@ -58,8 +57,7 @@
         $to_format = 'Y-m-d\TH:i:s';
         break;
       default:
-        // Set an invalid format which forces a MigrateException in format_date.
-        $from_format = '';
+        throw new MigrateException(sprintf('Field %s is an unknown date field type.', var_export($data['type'], TRUE)));
     }
     $process = [
       'value' => [
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/datetime/tests/src/Unit/process/DateFieldTest.php
@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\Tests\datetime\Unit\Plugin\migrate\field\d6;
+
+use Drupal\datetime\Plugin\migrate\field\d6\DateField;
+use Drupal\migrate\MigrateException;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @group migrate
+ */
+class DateFieldTest extends UnitTestCase {
+
+  /**
+   * @var \Drupal\migrate_drupal\Plugin\MigrateFieldInterface
+   */
+  protected $plugin;
+
+  /**
+   * @var \Drupal\migrate\Plugin\MigrationInterface
+   */
+  protected $migration;
+
+  /**
+   * Tests an Exception is thrown when the field type is not a known date type.
+   */
+  public function testUnknownDateType() {
+    $this->migration = $this->prophesize('Drupal\migrate\Plugin\MigrationInterface')->reveal();
+    $this->plugin = new DateField([], '', []);
+
+    $this->setExpectedException(MigrateException::class, "Field 'timestamp' is an unknown date field type.");
+    $this->plugin->processFieldValues($this->migration, '', ['type' => 'timestamp']);
+  }
+
+}