php - Deserialize JSON to EntityObject error with DateTimeImmutable

I need to deserialize a JSON file (which is a table from an old database) into a "Training" entity object.

The problem I am encountering is an error :

Failed to denormalize attribute "start_at" value for class "App\Entity\Training": Expected argument of type "DateTimeImmutable", "string" given at property path "start_at".

The serializer is called in the construct: (What is commented is a test without using the SerializerInterface)

public function __construct(
    string $projectDir,
    SerializerInterface $serializer,
    ManagerRegistry $doctrine
) {
    $this->projectDir = $projectDir;
    $this->serializer = $serializer;
    $this->doctrine = $doctrine;
    // $encoders = [new JsonEncoder()];
    // $normalizers = [new ObjectNormalizer(null, null, null, new ReflectionExtractor()), new DateTimeNormalizer()];
    // $this->serializer = new Serializer($normalizers, $encoders);
}

JSON file:

{
"id": "13",
"type_id": "3",
"title": "Technical training",
"description": "the description",
"start_at": "2018-05-14 09:00:00",
"end_at": "2018-05-18 17:00:00",
"pj": "file.pdf",
"places_dispos": "2",
"created_at": "2018-01-15 10:20:59",
"updated_at": "2019-04-29 18:01:55"
}

My function:

public function readJsonFile(string $fileName, string $tableName)
    {
        $file = $this->projectDir . '/var/json-database/extranet_table_' . $fileName . '.json';
        $fileString = file_get_contents($file);
        $fileObj = json_decode($fileString, true);

    return $this->convertJsonToObject($fileObj['data'], $tableName);
}

private function convertJsonToObject(array $arrayData, string $tableName)
    {
        $arrayTableObj = [];
        foreach ($arrayData as $data) {
            $data['id'] = intval($data['id']);

            //Boolean
            if (isset($data['active'])) {
                ($data['active'] === '1') ? $data['active'] = true : $data['active'] = false;
            }

            if (isset($data['updatable'])) {
                ($data['updatable'] === '1') ? $data['updatable'] = true : $data['updatable'] = false;
            }

            $dataJson = json_encode($data);
            // $dataObj = $this->serializer->deserialize($dataJson, "App\Entity\\". $tableName, 'json');
            $dataObj = $this->serializer->deserialize($dataJson, "App\Entity\\". $tableName, 'json', [
            DateTimeNormalizer::FORMAT_KEY => 'Y-m-d H:i:s',
        ]);
            $dataObj->setId($data['id']);

            $arrayTableObj[] = $dataObj;
        }

        return $this->importInDatabase($arrayTableObj);
    }

In my Training entity:

#[ORM\Column(type: 'datetime_immutable')]
private $startAt;

#[ORM\Column(type: 'datetime_immutable')]
private $endAt;

I tried to transform the content of "start_at" into DateTimeImmutable or DateTime, it doesn't change anything except the error message (array given).

Solution:

I think I understood my mistake. The displayed error did not help me. The problem I think, is that the property "start_at" in base is different from my entity "startAt".

If I rename "start_at" in the JSON to "startAt" it works. I don't delete the question, if it can help people not to make the same mistake as me.