fix typehint

Account/Account.php

@@ -435,11 +435,11 @@ class Account implements \JsonSerializable, ArrayableInterface
     /**
      * Get created at.
      *
-     * @return \DateTimeInterface
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }

System/File/ContainerInterface.php

@@ -124,11 +124,11 @@ interface ContainerInterface
     /**
      * Get the datetime when the resource got created.
      *
-     * @return \DateTime
+     * @return \DateTimeImmutable
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTimeInterface;
+    public function getCreatedAt() : \DateTimeImmutable;
 
     /**
      * Get the datetime when the resource got last modified.

System/File/Ftp/FileAbstract.php

@@ -151,7 +151,7 @@ abstract class FileAbstract implements ContainerInterface
     /**
      * {@inheritdoc}
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }

System/File/Local/FileAbstract.php

@@ -151,7 +151,7 @@ abstract class FileAbstract implements ContainerInterface
     /**
      * {@inheritdoc}
      */
-    public function getCreatedAt() : \DateTimeInterface
+    public function getCreatedAt() : \DateTimeImmutable
     {
         return $this->createdAt;
     }