diff -u b/core/modules/datetime/src/Plugin/migrate/field/DateField.php b/core/modules/datetime/src/Plugin/migrate/field/DateField.php
--- b/core/modules/datetime/src/Plugin/migrate/field/DateField.php
+++ b/core/modules/datetime/src/Plugin/migrate/field/DateField.php
@@ -31,6 +31,8 @@
     return [
       'date_default' => 'datetime_default',
       'format_interval' => 'datetime_time_ago',
+      // The date_plain formatter exists in Drupal 7 but not Drupal 6. It is
+      // added here because this plugin is declared for Drupal 6 and Drupal 7.
       'date_plain' => 'datetime_plain',
     ];
   }