How to modify jsonapi response?
You can change the JSON output of a date field by implementing a custom Normalizer for date fields in a custom module. I got it to work using the following code. This is a bit of a hatchet job, so you'll want to clean up and stuff, but otherwise it should do the job:

/src/Normalizer/DateTimeCustomNormalizer.php:

&lt;?php

namespace Drupal\custom_date_serializer\Normalizer;

use Drupal\serialization\Normalizer\DateTimeIso8601Normalizer;

/**
 * @internal
 */
class DateTimeCustomNormalizer extends DateTimeIso8601Normalizer {

  /**
   * {@inheritdoc}
   */
  protected $allowedFormats = [
    'RFC 3339' =&gt; \DateTime::RFC3339,
    'ISO 8601' =&gt; \DateTime::ISO8601,
    // used to be 'Y-m-d', so you can use any PHP date format you want
    // I just set it to this because it was easy to test
    'date-only' =&gt; \DateTime::ISO8601, 
  ];
}


And /custom_date_serializer.services.yml:

services:
  custom_date_serializer.normalizer.datetimecustom:
    class: \Drupal\custom_date_serializer\Normalizer\DateTimeCustomNormalizer
    arguments: ['@config.factory']
    tags:
      # Priority must be higher than other datetime normalizers.
      - { name: normalizer, priority: 40 }


And you'll need an info.yml obviously.