Добавил тест

tests/unit/Converter/FillTest.php

@@ -78,6 +78,7 @@ class FillTest extends \Codeception\Test\Unit
             new CollectionItem('2'),
         ];
         $helloRequest->collection = $collection;
+        $helloRequest->createdAt = new \DateTimeImmutable('2020-05-21 13:36:22');
 
         $dto = $dto2DataConverter->convert($helloRequest);
         // codecept_debug(json_encode($dto));
@@ -127,6 +128,7 @@ class FillTest extends \Codeception\Test\Unit
                     'value' => '2',
                 ],
             ],
+            'createdAt' => '2020-05-21T13:36:22+00:00'
         ], $dto);
     }
 }

tests/unit/Converter/fixtures/FillTest/HelloRequest.php

@@ -4,6 +4,7 @@ namespace Rinsvent\DTO2Data\Tests\unit\Converter\fixtures\FillTest;
 
 use Rinsvent\DTO2Data\Attribute\DataPath;
 use Rinsvent\DTO2Data\Attribute\PropertyPath;
+use Rinsvent\DTO2Data\Transformer\DateTimeFormat;
 use Rinsvent\DTO2Data\Transformer\Trim;
 
 #[HelloSchema]
@@ -22,4 +23,6 @@ class HelloRequest
     #[PropertyPath(path: 'uuid.id')]
     public UUID $uuid;
     public Collection $collection;
+    #[DateTimeFormat]
+    public \DateTimeImmutable $createdAt;
 }

tests/unit/Converter/fixtures/FillTest/HelloSchema.php

@@ -30,5 +30,6 @@ class HelloSchema extends Schema
         'collection' => [
             'value'
         ],
+        'createdAt'
     ];
 }