diff --git a/Models/Project.php b/Models/Project.php
index 1742156..3d23fa5 100755
--- a/Models/Project.php
+++ b/Models/Project.php
@@ -158,7 +158,7 @@ class Project
      * @var \DateTime
      * @since 1.0.0
      */
-    private \DateTime $createdAt;
+    private \DateTimeImmutable $createdAt;
 
     /**
      * Created by.
@@ -190,7 +190,7 @@ class Project
         $this->end->modify('+1 month');
 
         $this->endEstimated = clone $this->end;
-        $this->createdAt    = new \DateTime('now');
+        $this->createdAt    = new \DateTimeImmutable('now');
         $this->createdBy    = new NullAccount();
 
         $this->calendar = new Calendar();
@@ -699,7 +699,7 @@ class Project
      *
      * @since 1.0.0
      */
-    public function getCreatedAt() : \DateTime
+    public function getCreatedAt() : \DateTimeInterface
     {
         return $this->createdAt;
     }
diff --git a/Models/ProjectMapper.php b/Models/ProjectMapper.php
index 050a1ae..16fd2dc 100755
--- a/Models/ProjectMapper.php
+++ b/Models/ProjectMapper.php
@@ -52,7 +52,7 @@ final class ProjectMapper extends DataMapperAbstract
         'projectmanagement_project_progress'        => ['name' => 'projectmanagement_project_progress',        'type' => 'int',          'internal' => 'progress'],
         'projectmanagement_project_progress_type'   => ['name' => 'projectmanagement_project_progress_type',   'type' => 'int',          'internal' => 'progressType'],
         'projectmanagement_project_created_by'      => ['name' => 'projectmanagement_project_created_by',      'type' => 'int',          'internal' => 'createdBy', 'readonly' => true],
-        'projectmanagement_project_created_at'      => ['name' => 'projectmanagement_project_created_at',      'type' => 'DateTime',     'internal' => 'createdAt', 'readonly' => true],
+        'projectmanagement_project_created_at'      => ['name' => 'projectmanagement_project_created_at',      'type' => 'DateTimeImmutable', 'internal' => 'createdAt', 'readonly' => true],
     ];
 
     /**