How to use $dates array and Factory together

I have a factory with the below definition

  public function definition()
  {
    $startDate = Carbon::createFromFormat('Y', $this->faker->year);
    $endDate = $startDate->addYear();

    return [
      'name' => $startDate->year,
      'start_date' => $startDate->format('Y-m-d'),
      'end_date' => $endDate->format('Y-m-d'),
    ];
  }

So this generates an array like below

[
  'name' => 2020,
  'start_date' => '2020-01-30',
  'end_date' => '2021-01-30'
]

The above worked as expected until a requirement to return the start_date field and the end_date field arose. So in my model I added

  protected $dates = ['created_at', 'updated_at', 'start_date', 'end_date'];

In my validators, I validate that the format is Y-m-d

  public function rules()
  {
    return [
      'name' => 'required|unique:academic_years',
      'start_date' => 'required|date_format:Y-m-d|before_or_equal:end_date',
      'end_date' => 'required|date_format:Y-m-d',
    ];
  }

I also have the below test

  public function should_throw_error_if_date_format_is_invalid()
  {
    Permission::factory()->state(['name' => 'create academic year'])->create();
    $user = User::factory()->create();
    $user->givePermissionTo('create academic year');
    $this->actingAs($user, 'api')->postJson('/api/academic-years', [
      'name' => $this->faker->year,
      'start_date' => '01-01-2019',
      'end_date' => '2019-01-01'
    ])->assertStatus(422);
    $this->actingAs($user, 'api')->postJson('/api/academic-years', [
      'name' => $this->faker->year,
      'end_date' => '01-01-2019',
      'start_date' => '2019-01-01'
    ])->assertStatus(422);
  }

The test fails, but If I remove the $dates array, the test passes. On checking the object passed into the request I see a different date format {"name":1995,"start_date":"1995-04-04T00:00:00.000000Z","end_date":"1995-04-04T00:00:00.000000Z"} from the format provided in the Factory

How can I resolve this