When using faker geting Unknown Formatter error

I have a container factory which is as follows:

<?php

namespace Database\Factories;

use App\Models\Container;
use Illuminate\Database\Eloquent\Factories\Factory;
use Illuminate\Support\Arr;

class ContainerFactory extends Factory
{
    /**
     * Define the model's default state.
     *
     * @return array
     */
    public function definition()
    {
         $preparationDate = $this->faker->dateTimeBetween('-5 years', '-3 weeks');
         .
         .
         .
         return [
             .
             .
             'preparation_date' => $preparationDate,
          ];
     }
}

When I use tinker, this produces an appropriate container.

However, when I use it in a test class method (i.e. not a test but a helper function) like:

namespace Tests\Feature\Samples;

use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Foundation\Testing\WithFaker;
use Tests\TestCase;

class FormValidationTest extends TestCase
{

   /* Number of test cases here not shown
    *
   */
    
   // This function is a helper for my data provider for my tests.
    public function generatePostData(string $path, $value)
    {
        $container = Container::factory()->create(); // This line gives error
          .
          .
          . 
         
          return [ 
                .
                .
                .
               'container' => $container
           ];
      }
}

It throws the error:

  The data provider specified for Tests\Feature\Samples\FormValidationTest ::test_user_cant_store_invalid_samples is invalid.
InvalidArgumentException: Unknown formatter "dateTimeBetween"

Edit: The same $container = Container::factory()->create(); works fine within a test method in the same file like

public function test_a_feature() {
   $container = Container::factory()->create(); // This works when put in a test
}

How can I create models in a separate function?