diff --git a/src/pvv/side/agenda.php b/src/pvv/side/agenda.php
index 629e0dc..191e2b5 100644
--- a/src/pvv/side/agenda.php
+++ b/src/pvv/side/agenda.php
@@ -21,7 +21,7 @@ class Agenda {
     $this->activities = $activities;
   }
 
-  public static function getFormattedDate(\DateTime $date): string {
+  public static function getFormattedDate(\DateTimeImmutable $date): string {
     return $date->format('l j. M H.i');
   }