@dunglas , seems no one has this trouble，how do you solve this please?

@videni I have exactly the same problem. Were you able to solve this?

@Lobosque , I create a dedicated denormalizer which will unset the key of the date time attribute if user submit an empty date time. as the example below, currently I hard coded the key 'expires_at' for I only have one entity to deal with, if you have several attributes you can add configuration yourself.

use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;

class EmptyDateTimeDenormalizer implements DenormalizerInterface
{
    private $decorated;

    public function __construct(
        ItemNormalizer $decorated
    ) {
        $this->decorated = $decorated;
    }

    public function denormalize($data, $class, $format = null, array $context = [])
    {
        $data = $this->removeEmptyDateTime($data, $class);

        return $this->decorated->denormalize($data, $class, $format, $context);
    }

    private function removeEmptyDateTime($data, $type)
    {
        if ($type == User::class && isset($data['expires_at']) && $data['expires_at'] == '') {
            unset($data['expires_at']);
        }

        return $data;
    }
}

@videni thank you for sharing your solution.
Even though we can work around the problem with a custom denormalizer, I still think this it should accept empty values and leave the validator to handle if it is valid or not.

This is happening to me too. Can't find a way to solve this.
I'll try your solution @videni, thank you!

@dunglas Would you accept a PR to make API Platform accept empty datetime and denormalize them as null?

Hey Guys,

I have handle it that way: https://stackoverflow.com/questions/61852792/api-platform-datetimenormalizer-not-allowing-null/61852793#61852793

Same problem here at 2021.

Just set for setter function with ? and param DateTime|null. It works!

/**
* @param DateTime|null $dateAt
*/
public function setDateAt(?DateTime $dateAt): void
{
$this->dateAt = $dateAt;
}