diff --git a/src/pvv/side/repeatingactivity.php b/src/pvv/side/repeatingactivity.php
index 9ad3e7c..9bcda82 100644
--- a/src/pvv/side/repeatingactivity.php
+++ b/src/pvv/side/repeatingactivity.php
@@ -1,12 +1,12 @@
 <?php //declare(strict_types=1);
 namespace pvv\side;
 
-use \DateTime;
+use \DateTimeImmutable;
 
 interface RepeatingActivity extends Activity {
 
-	public function getNextEventFrom(DateTime $date) /* : Event */;
+	public function getNextEventFrom(DateTimeImmutable $date) /* : Event */;
 
-	public function getPreviousEventFrom(DateTime $date) /* : Event */;
+	public function getPreviousEventFrom(DateTimeImmutable $date) /* : Event */;
 
 }