diff --git a/Interfaces/GSD/Model/GSDCostCenter.php b/Interfaces/GSD/Model/GSDCostCenter.php
index dff7e4b..8bf7647 100755
--- a/Interfaces/GSD/Model/GSDCostCenter.php
+++ b/Interfaces/GSD/Model/GSDCostCenter.php
@@ -75,11 +75,11 @@ final class GSDCostCenter implements \JsonSerializable
     }
 
     /**
-     * @return \DateTime
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt ?? new \DateTime();
     }
diff --git a/Interfaces/GSD/Model/GSDCostObject.php b/Interfaces/GSD/Model/GSDCostObject.php
index 3242489..cd12e06 100755
--- a/Interfaces/GSD/Model/GSDCostObject.php
+++ b/Interfaces/GSD/Model/GSDCostObject.php
@@ -75,11 +75,11 @@ final class GSDCostObject implements \JsonSerializable
     }
 
     /**
-     * @return \DateTime
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt ?? new \DateTime();
     }
diff --git a/Models/ExchangeLog.php b/Models/ExchangeLog.php
index 30d044f..3181e68 100644
--- a/Models/ExchangeLog.php
+++ b/Models/ExchangeLog.php
@@ -167,11 +167,11 @@ class ExchangeLog implements \JsonSerializable, ArrayableInterface
     /**
      * Get created at.
      *
-     * @return array
+     * @return DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterfaceInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }