diff --git a/Interfaces/GSD/Model/GSDCostCenter.php b/Interfaces/GSD/Model/GSDCostCenter.php
index f78aafd..da25261 100644
--- a/Interfaces/GSD/Model/GSDCostCenter.php
+++ b/Interfaces/GSD/Model/GSDCostCenter.php
@@ -31,7 +31,7 @@ final class GSDCostCenter implements \JsonSerializable
      * @var int
      * @since 1.0.0
      */
-    protected $id = 0;
+    protected int $id = 0;
 
     /**
      * Creator.
@@ -47,7 +47,7 @@ final class GSDCostCenter implements \JsonSerializable
      * @var null|\DateTime
      * @since 1.0.0
      */
-    protected $createdAt = null;
+    protected ?\DateTime $createdAt = null;
 
     /**
      * Description.
@@ -55,7 +55,7 @@ final class GSDCostCenter implements \JsonSerializable
      * @var string
      * @since 1.0.0
      */
-    protected $description = '';
+    protected string $description = '';
 
     /**
      * Cost center.
@@ -63,7 +63,7 @@ final class GSDCostCenter implements \JsonSerializable
      * @var string
      * @since 1.0.0
      */
-    protected $costCenter = '';
+    protected string $costCenter = '';
 
     /**
      * Constructor.
diff --git a/Interfaces/GSD/Model/GSDCostObject.php b/Interfaces/GSD/Model/GSDCostObject.php
index 3ebe435..6e46ef7 100644
--- a/Interfaces/GSD/Model/GSDCostObject.php
+++ b/Interfaces/GSD/Model/GSDCostObject.php
@@ -31,7 +31,7 @@ final class GSDCostObject implements \JsonSerializable
      * @var int
      * @since 1.0.0
      */
-    protected $id = 0;
+    protected int $id = 0;
 
     /**
      * Creator.
@@ -47,7 +47,7 @@ final class GSDCostObject implements \JsonSerializable
      * @var null|\DateTime
      * @since 1.0.0
      */
-    protected $createdAt = null;
+    protected ?\DateTime $createdAt = null;
 
     /**
      * Description.
@@ -55,7 +55,7 @@ final class GSDCostObject implements \JsonSerializable
      * @var string
      * @since 1.0.0
      */
-    protected $description = '';
+    protected string $description = '';
 
     /**
      * Cost object.
@@ -63,7 +63,7 @@ final class GSDCostObject implements \JsonSerializable
      * @var string
      * @since 1.0.0
      */
-    protected $costObject = '';
+    protected string $costObject = '';
 
     /**
      * Constructor.
diff --git a/Models/InterfaceManager.php b/Models/InterfaceManager.php
index dd04561..6da73d9 100644
--- a/Models/InterfaceManager.php
+++ b/Models/InterfaceManager.php
@@ -35,7 +35,7 @@ class InterfaceManager
      * @var int
      * @since 1.0.0
      */
-    private $id = 0;
+    private int $id = 0;
 
     /**
      * File path.
@@ -43,7 +43,7 @@ class InterfaceManager
      * @var string
      * @since 1.0.0
      */
-    private $path = '';
+    private string $path = '';
 
     /**
      * Info data.