diff -u b/core/lib/Drupal/Component/Datetime/DateTimePlus.php b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
--- b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
+++ b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
@@ -287,6 +287,7 @@
    * {@inheritdoc}
    */
   public function __call($method, $args) {
+    // @todo consider using assert() as per https://www.drupal.org/node/2451793.
     if (!isset($this->dateTimeObject)) {
       throw new \Exception('DateTime object not set.');
     }