Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/validator/Constraints/DateTime.php

@@ -0,0 +1,37 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Validator\Constraints;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * @Annotation
+ * @Target({"PROPERTY", "METHOD", "ANNOTATION"})
+ *
+ * @author Bernhard Schussek <bschussek@gmail.com>
+ *
+ * @api
+ */
+class DateTime extends Constraint
+{
+    const INVALID_FORMAT_ERROR = 1;
+    const INVALID_DATE_ERROR = 2;
+    const INVALID_TIME_ERROR = 3;
+
+    protected static $errorNames = array(
+        self::INVALID_FORMAT_ERROR => 'INVALID_FORMAT_ERROR',
+        self::INVALID_DATE_ERROR => 'INVALID_DATE_ERROR',
+        self::INVALID_TIME_ERROR => 'INVALID_TIME_ERROR',
+    );
+
+    public $message = 'This value is not a valid datetime.';
+}