Symfony 5 and API Platform, always receiving "This value should be of type string" when validating datetime

Question

I tried to add validation to dateTime with symfony Assert and API platform :

use Symfony\Component\Validator\Constraints as Assert;

denormalizationContext={"disable_type_enforcement"=true}


/**
 * @ORM\Column(type="datetime")
 * @Groups({"invoices_read", "customers_read", "invoices_subresource"})
 * @Assert\NotBlank(message="La date de la facture doit être renseignée !")
 * @Assert\DateTime(message="La date doit être au format YYYY-MM-DD")
 */
private $sentAt;

I tried to post data with postman . I always receive :

{
    "@context": "/api/contexts/ConstraintViolationList",
    "@type": "ConstraintViolationList",
    "hydra:title": "An error occurred",
    "hydra:description": "sentAt: This value should be of type string.",
    "violations": [
        {
            "propertyPath": "sentAt",
            "message": "This value should be of type string."
        }
    ]
}