My first approach to Drupal Commerce: the standard for the robust enterprise e-commerce solution. Loved it!

product_variation_name:
  enabled:         # boolean
  times:
    start_time:    # time
    end_time:      # time
    free_shipping: # boolean
    UUID:          # a random string
 
 # this for each product variations.
 # [...]

<?php

# [...]

  /**
   * 
   * Check the two time periods overlap.
   *
   * Example:
   * $periods = [
   *    ['start_time' => "09:00", 'end_time' => '10:30'],
   *    ['start_time' => "14:30", "end_time" => "16:30"],
   *    ['start_time' => "11:30", "end_time" => "13:00"],
   *    ['start_time' => "10:30", "end_time" => "11:30"],
   * ];
   *
   * @param $periods
   * @param string $start_time_key
   * @param string $end_time_key
   * @param int $corrupted
   * @return bool
   *
   * Check the code on this page:
   *  - https://dev.to/xichlo/determine-whether-two-date-ranges-overlap-in-php-1b5a
   * 
   * Of course I understood the code that I copy&paste, and I added
   * the parameter $corrupted because I need the position of the error.
   *
   */
  private function isTimesOverlapped($periods, $start_time_key, $end_time_key, &$corrupted = null) {
    usort($periods, function ($a, $b) use ($start_time_key, $end_time_key) {
      return strtotime($a[$start_time_key]) <=> strtotime($b[$end_time_key]);
    });

    foreach ($periods as $key => $period) {
      if ($key != 0) {
        if (strtotime($period[$start_time_key]) < strtotime($periods[$key - 1][$end_time_key])) {
          $corrupted = $key - 1;
          return true;
        }
      }
    }

    return false;
  }

<?php

namespace Drupal\price_in_time\Resolvers;

# Drupal Commerce.
use Drupal\commerce\Context;
use Drupal\commerce\PurchasableEntityInterface;
use Drupal\commerce_price\Resolver\PriceResolverInterface;
use Drupal\commerce_price\Price;

# Drupal Entity.
use Drupal\user\Entity\User;

# Drupal Core.
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Database\Connection;
use Drupal\Core\Datetime\DrupalDateTime;

/**
 * Check for price adjustment based on time slot (and
 * current time).
 */
class PriceResolver implements PriceResolverInterface {

  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $config_factory;

  /**
   * The database connection.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $connection;

  /**
   * Constructs a Price Resolver object.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory.
   *
   * @param \Drupal\Core\Database\Connection $connection
   *   The database connection.
   */
  final public function __construct(ConfigFactoryInterface $config_factory, Connection $connection) {
    $this->config_factory = $config_factory->get('price_in_time.settings');
    $this->connection = $connection;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('database'),
    );
  }

  /**
   * {@inheritdoc}
   */
  public function resolve(PurchasableEntityInterface $entity, $quantity, Context $context) {

    $query = $this->connection
      ->select('commerce_product_variation_slots_data', 'commerce_product_slots')
      ->fields('commerce_product_slots', [
        'product_variation_sku',
        'number',
        'currency_code',
        'slot_uuid',
      ])
      ->condition('commerce_product_slots.product_variation_sku', $entity->getSku())
      ->execute()
      ->fetchAllAssoc('slot_uuid');

    if ($this->config_factory->get($entity->bundle() . '.enabled') && !empty($query)) {
      return $this->getCurrentTimeSlotPrice(
        $query,
        $entity->bundle()
      );
    } return $entity->getPrice();
  }

  /**
   *
   * @param array $results
   * @param string $bundle
   * @return Price
   *
   * Return the price of the specific slot of time (based on the
   * current time). This will return the price everytime because
   * we check the existence of the record before.
   *
   */
  private function getCurrentTimeSlotPrice($results, $bundle) {
    $bundle_settings = $this->config_factory->get($bundle);
    $current_time = strtotime(date('H:i:s'));

    foreach ($bundle_settings['times'] as $key => $slot) {
      if ($current_time >= strtotime($slot['start']) && $current_time <= strtotime($slot['end'])) {
        if (isset($results[$slot['uuid']])) {
          return new Price(
            $results[$slot['uuid']]->number,
            $results[$slot['uuid']]->currency_code
          );
        }
      }
    }
  }
}

price_in_time__free_shipping:
  label: 'Price in Time: free shipping'
  singular_label: 'price in time free shipping adjustment'
  plural_label: 'price in time free shipping adjustments'
  has_ui: true
  weight: 10

<?php

namespace Drupal\price_in_time\OrderProcessor;

# Drupal Commerce.
use Drupal\commerce_order\Adjustment;
use Drupal\commerce_order\Entity\OrderInterface;
use Drupal\commerce_order\OrderProcessorInterface;

# Drupal Core.
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Database\Connection;
use Drupal\Core\Datetime\DrupalDateTime;

/**
 * Check for free shipping during order processing (based on
 * product variation included into basket).
 */
class FreeShippingOrderProcessor implements OrderProcessorInterface {

  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $config_factory;

  /**
   * The database connection.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $connection;

  /**
   * Constructs an Order Processor object.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory.
   *
   * @param \Drupal\Core\Database\Connection $connection
   *   The database connection.
   */
  final public function __construct(ConfigFactoryInterface $config_factory, Connection $connection) {
    $this->config_factory = $config_factory->get('price_in_time.settings');
    $this->connection = $connection;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('database'),
    );
  }

  /**
   * {@inheritdoc}
   */
  public function process(OrderInterface $order) {

    $shipment_referenced_entities = $order->shipments->referencedEntities();

    if (!empty($shipment = reset($shipment_referenced_entities)) &&
        $shipment->getAmount()->getNumber() > 0 &&
        $this->checkForFreeShippingTime($order->getItems())) {

      $order->addAdjustment(new Adjustment([
        'type' => 'price_in_time__free_shipping',
        'label' => t('Free shipping bonus'),
        'amount' => $shipment->getAmount()->multiply('-1'),
      ]));
    }
  }

  /**
   *
   * @param array $order_items
   * @return bool
   *
   * This will return true if the free shipping is set for the specific
   * slot of time, otherwise false (will not apply the bonus on checkout).
   *
   */
  private function checkForFreeShippingTime($order_items) {

    foreach ($order_items as $key => $item) {
      $purchased_referenced_entities = $item->purchased_entity->referencedEntities();
      $item_entity = reset($purchased_referenced_entities);
      $item_bundle = $item_entity->bundle();
      $item_settings = $this->config_factory->get($item_bundle);
      $current_time = strtotime(date('H:i:s'));

      foreach ($item_settings['times'] as $key => $slot) {
        if ($current_time >= strtotime($slot['start']) && $current_time <= strtotime($slot['end'])) {
          if ($slot['free_shipping']) return true;
        }
      }
    }

    return false;
  }
}