services:
  event_countdown.date_calculator:
    class: Drupalevent_countdownDateCalculator

event_countdown.content:
  defaults:
    _title: 'Event Countdown'
  requirements:
    _permission: 'access content'
  options:
    no_cache: 'TRUE'

<?php

namespace Drupalevent_countdown;

class DateCalculator {

  // forms the block's output based on given date
  public function daysUntilEventStarts($date) {
    // get difference in days between now and the given date
    $difference = $this->getDifferenceInDaysFromCurrentDate($date);
      // if event date is in the future
      if($difference >= 1) {
        return "Days left until the event starts: " . $difference;
      // if event date and current date are on the same day
      } else if($difference == 0) {
        return "The event is in progress.";
      // if event date is in the past
      } else {
        return "The event has already ended.";
      }
  }

  // calculates difference in days between now and given date
  public function getDifferenceInDaysFromCurrentDate($date) {
    // current time
    $now = time();
    // event datetime field converted to timestamp
    $event_date = strtotime($date);
    // timestamp difference
    $difference = $event_date - $now;
    // timestamp difference rounded down to days
    return round($difference / (60 * 60 * 24));
  }

}

<?php

namespace Drupalevent_countdownPluginBlock;

use DrupalCoreBlockBlockBase;
use SymfonyComponentDependencyInjectionContainerInterface;
use Drupalevent_countdownDateCalculator;


/**
 * Provides an event countdown block.
 *
 * @Block(
 *   id = "event_countdown_block",
 *   admin_label = @Translation("Event countdown block"),
 *   category = @Translation("Event countdown block"),
 * )
 */

class EventCountdownBlock extends BlockBase {

  protected $dateCalculator;

   public function __construct(DateCalculator $dateCalculator) {
     $this->dateCalculator = $dateCalculator;
   }

   public static function create(ContainerInterface $container) {
     return new static($container->get('event_countdown.date_calculator'));
   }

   public function build() {

      // get current node based on route
      $node = Drupal::routeMatch()->getParameter('node');

      //check whether node type equals 'event' - if it does, get text output, otherwise display an error message
      if($node->getType() == "event") {
        // get datetime value from node
        $date = $node->field_event_date->value;
        // call service method to calculate difference in days
        $output = $this->dateCalculator->daysUntilEventStarts($date);
      } else {
        // display error
        $output = "Woops! This block is intended only for event pages.";
      }

      return array(
        // output
        '#markup' => $output,
        // prevent block caching
        '#cache' => [
          'max-age' => 0,
          ],
      );
  }

}