Module.php

<?php

namespace modules;

use Craft;
use craft\elements\Entry;
use craft\events\ModelEvent;
use DateTime;
use yii\base\Event;

class Module extends \yii\base\Module
{
    /**
     * Initializes the module.
     */
    public function init()
    {
        // Set a @modules alias pointed to the modules/ directory
        Craft::setAlias('@modules', __DIR__);

        // Set the controllerNamespace based on whether this is a console or web request
        if (Craft::$app->getRequest()->getIsConsoleRequest()) {
            $this->controllerNamespace = 'modules\\console\\controllers';
        } else {
            $this->controllerNamespace = 'modules\\controllers';
        }

        parent::init();

        // Custom initialization code goes here...
        Event::on(Entry::class, Entry::EVENT_BEFORE_SAVE, [$this, 'autoExpireNewsEntries']);
    }

    /**
     * Sets the expiration date for News entries on element save.
     *
     * @param ModelEvent $event
     */
    public function autoExpireNewsEntries(ModelEvent $event)
    {
        $entry = $event->sender;

        // Exit early if the entry being saved is not in the section or entry type
        // that we are targeting
        if (
            !in_array($entry->sectionId, [4]) ||
            !in_array($entry->getType()->id, [4, 5])
        ) {
            return;
        }

        // Exit early if the field is not empty. Remove this if you want to override
        // whatever is set in the edit entry form
        if ($entry->expiryDate !== null) {
            return;
        }

        // Calculate the new date
        // $newDate = (new DateTime())->modify('+7 days 04:00');
        // $newDate = (clone $entry->postDate)->modify('first day of next month 05:00');
        // $newDate = $entry->myFieldHandle !== null ? $entry->myFieldHandle : $entry->postDate;
        $newDate = new DateTime('2018-04-23 09:00');

        // Set the new date. Element attributes like postDate and expiryDate can
        // be set directly, if you want to set a custom date field here use
        // $entry->setFieldValue('myFieldHandle', $newDate);
        $entry->expiryDate = $newDate;
    }
}