diff --git a/.gitignore b/.gitignore
index b98b4400..fe1abf22 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
 vendor/
 composer.lock
 /.php-cs-fixer.cache
+/.phpunit.cache
+.phpunit.result.cache
diff --git a/.jane-openapi b/.jane-openapi
index 85e86f26..eba467cc 100644
--- a/.jane-openapi
+++ b/.jane-openapi
@@ -7,5 +7,5 @@ return [
   'openapi-file'          => __DIR__ . '/spec/v1.45.json',
   'reference'             => true,
   'strict'                => false,
-  'date-input-format'     => 'Y-m-d\TH:i:s.uuP',
+  'date-input-format'     => '',
 ];
diff --git a/composer.json b/composer.json
index cd5f9f6a..f027ef4e 100644
--- a/composer.json
+++ b/composer.json
@@ -32,6 +32,7 @@
         "ergebnis/composer-normalize": "^2.42",
         "friendsofphp/php-cs-fixer": "^3.8",
         "jane-php/open-api-3": "^7.4",
+        "phpunit/phpunit": "^11.0",
         "roave/security-advisories": "dev-latest"
     },
     "conflict": {
@@ -44,6 +45,11 @@
             "Docker\\API\\": "src/"
         }
     },
+    "autoload-dev": {
+        "psr-4": {
+            "Docker\\API\\Tests\\": "tests/"
+        }
+    },
     "config": {
         "allow-plugins": {
             "ergebnis/composer-normalize": true,
@@ -63,6 +69,7 @@
         }
     },
     "scripts": {
+        "test": "vendor/bin/phpunit",
         "generate": [
             "vendor/bin/jane-openapi generate",
             "cat patches/*.patch | patch -s -p1"
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
new file mode 100644
index 00000000..6e1a5d82
--- /dev/null
+++ b/phpunit.xml.dist
@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+>
+    <testsuites>
+        <testsuite name="Tests">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+    <source>
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/src/Normalizer/HealthcheckResultNormalizer.php b/src/Normalizer/HealthcheckResultNormalizer.php
index b3962a2b..a4437632 100644
--- a/src/Normalizer/HealthcheckResultNormalizer.php
+++ b/src/Normalizer/HealthcheckResultNormalizer.php
@@ -33,18 +33,18 @@ public function supportsNormalization(mixed $data, ?string $format = null, array
 
     public function denormalize(mixed $data, string $type, ?string $format = null, array $context = []): mixed
     {
-        if (isset($data['$ref'])) {
+        $object = new \Docker\API\Model\HealthcheckResult();
+        if (null === $data || false === \is_array($data)) {
+            return $object;
+        }
+        if (isset($data['$ref']) && !isset($data['type']) && !isset($data['properties']) && !isset($data['allOf'])) {
             return new Reference($data['$ref'], $context['document-origin']);
         }
         if (isset($data['$recursiveRef'])) {
             return new Reference($data['$recursiveRef'], $context['document-origin']);
         }
-        $object = new \Docker\API\Model\HealthcheckResult();
-        if (null === $data || false === \is_array($data)) {
-            return $object;
-        }
         if (\array_key_exists('Start', $data) && null !== $data['Start']) {
-            $object->setStart(\DateTime::createFromFormat('Y-m-d\TH:i:s.uuP', $data['Start']));
+            $object->setStart('Z' === (new \DateTime($data['Start']))->getTimezone()->getName() ? (new \DateTime($data['Start']))->setTimezone(new \DateTimeZone('GMT')) : new \DateTime($data['Start']));
             unset($data['Start']);
         } elseif (\array_key_exists('Start', $data) && null === $data['Start']) {
             $object->setStart(null);
@@ -80,7 +80,7 @@ public function normalize(mixed $data, ?string $format = null, array $context =
     {
         $dataArray = [];
         if ($data->isInitialized('start') && null !== $data->getStart()) {
-            $dataArray['Start'] = $data->getStart()?->format('Y-m-d\TH:i:sP');
+            $dataArray['Start'] = $data->getStart()->format('Y-m-d\TH:i:sP');
         }
         if ($data->isInitialized('end') && null !== $data->getEnd()) {
             $dataArray['End'] = $data->getEnd();
diff --git a/tests/Normalizer/HealthcheckResultNormalizerTest.php b/tests/Normalizer/HealthcheckResultNormalizerTest.php
new file mode 100644
index 00000000..c0ba83d0
--- /dev/null
+++ b/tests/Normalizer/HealthcheckResultNormalizerTest.php
@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Docker\API\Tests\Normalizer;
+
+use Docker\API\Model\HealthcheckResult;
+use Docker\API\Normalizer\HealthcheckResultNormalizer;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+class HealthcheckResultNormalizerTest extends TestCase
+{
+    private HealthcheckResultNormalizer $normalizer;
+
+    protected function setUp(): void
+    {
+        $this->normalizer = new HealthcheckResultNormalizer();
+    }
+
+    public static function parsedStartProvider(): iterable
+    {
+        yield 'nanosecond precision with Z timezone (localstack format)' => [
+            '2019-12-22T10:59:05.6385933Z',
+            '2019-12-22',
+            '10:59:05',
+        ];
+        yield 'microsecond precision with numeric offset' => [
+            '2019-12-22T10:59:05.638593+00:00',
+            '2019-12-22',
+            '10:59:05',
+        ];
+    }
+
+    #[DataProvider('parsedStartProvider')]
+    public function testDenormalizeStartParsesDateTime(string $input, string $expectedDate, string $expectedTime): void
+    {
+        $result = $this->normalizer->denormalize(['Start' => $input], HealthcheckResult::class);
+
+        self::assertInstanceOf(\DateTimeInterface::class, $result->getStart());
+        self::assertSame($expectedDate, $result->getStart()->format('Y-m-d'));
+        self::assertSame($expectedTime, $result->getStart()->format('H:i:s'));
+    }
+
+    public static function nullStartProvider(): iterable
+    {
+        yield 'explicit null' => [['Start' => null]];
+        yield 'unparsable string' => [['Start' => 'not-a-date']];
+        yield 'non-string value' => [['Start' => 12345]];
+    }
+
+    #[DataProvider('nullStartProvider')]
+    public function testDenormalizeStartReturnsNull(array $data): void
+    {
+        $result = $this->normalizer->denormalize($data, HealthcheckResult::class);
+
+        self::assertNull($result->getStart());
+    }
+
+    public function testDenormalizeStartNotInitializedWhenKeyMissing(): void
+    {
+        $result = $this->normalizer->denormalize([], HealthcheckResult::class);
+
+        self::assertFalse($result->isInitialized('start'));
+    }
+}