Big Pickle (free)

<?php

namespace Drupal\current_date_block\Plugin\Block;

use Drupal\Core\Block\Attribute\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

#[Block(
  id: "current_date_block",
  admin_label: new TranslatableMarkup("Current date"),
  category: new TranslatableMarkup("Custom")
)]
class CurrentDateBlock extends BlockBase {

  public function defaultConfiguration(): array {
    return [
      'date_format' => 'Y-m-d H:i:s',
    ];
  }

  public function blockForm($form, FormStateInterface $form_state): array {
    $form['date_format'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Date format'),
      '#description' => $this->t('Enter a PHP date format string (e.g. @example). See <a href=":url">PHP date documentation</a>.', [
        '@example' => 'Y-m-d H:i:s',
        ':url' => 'https://www.php.net/manual/en/datetime.format.php',
      ]),
      '#default_value' => $this->configuration['date_format'],
      '#required' => TRUE,
    ];
    return $form;
  }

  public function blockSubmit($form, FormStateInterface $form_state): void {
    $this->configuration['date_format'] = $form_state->getValue('date_format');
  }

  public function blockValidate($form, FormStateInterface $form_state): void {
    $format = $form_state->getValue('date_format');
    $now = new \DateTime();
    $result = $now->format($format);
    if ($result === FALSE) {
      $form_state->setErrorByName('date_format', $this->t('Invalid date format string.'));
    }
  }

  public function build(): array {
    $now = new \DateTime();
    return [
      '#markup' => $this->t('Current date and time: @date', [
        '@date' => $now->format($this->configuration['date_format']),
      ]),
    ];
  }

}