diff --git a/Models/Calendar.php b/Models/Calendar.php
index 3a54433..f4e7bfd 100755
--- a/Models/Calendar.php
+++ b/Models/Calendar.php
@@ -196,11 +196,11 @@ class Calendar
     }
 
     /**
-     * @return \DateTime
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }
diff --git a/Models/Event.php b/Models/Event.php
index f5ea39a..8115971 100755
--- a/Models/Event.php
+++ b/Models/Event.php
@@ -238,11 +238,11 @@ class Event
     }
 
     /**
-     * @return \DateTime
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }