diff --git a/core/tests/bootstrap.php b/core/tests/bootstrap.php
index 08d8871..9394707 100644
--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -25,3 +25,7 @@
 // numbers handling.
 // @see drupal_environment_initialize()
 setlocale(LC_ALL, 'C');
+
+// Set the default timezone. While this doesn't cause any tests to fail, PHP
+// complains if 'date.timezone' is not set in php.ini.
+date_default_timezone_set('UTC');