DateTimePicker and Datatransformer, the form render doesnt take the string into account

I got somme issue with Symfony to convert a DateTime into string. I use a DataTransformer to format my Datetime but in the form, there is an error that say : “This value should be of type string”.

Here is my code:
My Entity : Shift.php (only the necessary)

 * @ORMId()
 * @ORMGeneratedValue()
 * @ORMColumn(type="integer")
private $id;
 * @ORMColumn(type="datetime")
 * @AssertDateTime(message="La date de début doit être au format DateTime")
private $start_at;

My ShiftType :

    ->add('start_at', TextType::class, ['attr' => [ 'class' => 'dateTimePicker']])
    ->add('end_at', TextType::class, ['attr' => [ 'class' => 'dateTimePicker']])

And my DataTransformer :

 * @param DateTime|null $datetime
 * @return string
public function transform($datetime)
    if ($datetime === null) 
        return '';
    return $datetime->format('Y-m-d H:i');
 * @param string $dateString
 * @return Datetime|null
public function reverseTransform($dateString)
    if (!$dateString) 
        throw new TransformationFailedException('Pas de date(string) passé');
    $date = Datetime::createFromFormat('Y-m-d H:i', $dateString);
    if($date === false){
        throw new TransformationFailedException("Le format n'est pas le bon (fonction reverseTransform)" . "$dateString");
    return $date;

As i said, when i want submit the form, there are errors with the form.
It said “This value should be of type string.” and it’s caused by :

SymfonyComponentValidatorConstraintViolation {#1107 ▼
  root: SymfonyComponentFormForm {#678 …}
  path: "data.start_at"
  value: DateTime @1578465000 {#745 ▶}

Something weard, when i want to edit a shift, Symfony get the date from the db and transform it into string with no error message. But as i want to save the edit, i got the same issue

Could you help me please ?

Source: Symfony Questions