diff --git a/api/src/DateTime.php b/api/src/DateTime.php
index db9b9b38d2..4fba95ef58 100644
--- a/api/src/DateTime.php
+++ b/api/src/DateTime.php
@@ -177,7 +177,7 @@ class DateTime extends \DateTime
 	 */
 	public function add($interval)
 	{
-		if (is_string($interval)) $interval = DateInterval::createFromDateString($interval);
+		if (is_string($interval)) $interval = \DateInterval::createFromDateString($interval);
 
 		parent::add($interval);
 	}