[Drupal 8.3.0] I'm trying to show a message for specific content type through a new module (I'm a beginner). To illustrate, here's a piece of code I have.

File test_module.module (updated code, still not working):

<?php

/**
 * @file
 * Implementation of hook_node_view();
 */

function test_module_node_view($node, $view_mode) {

  if ($view_mode == 'full' && $node->getType() == 'test_content') {

      /*Message with current date and time*/
      $current_time = time();
      drupal_set_message(t('Hello! Today is '.format_date($current_time,'medium')));
    }
  }

As far as I know, this should work, but it doesn't and I don't know why. The message is not showing for a node of content type "test_content". How can I fix this? Is there another way to alter nodes of custom content types?

[Update - some screenshots] (Test_content_example is a node of type 'test_content', so there should be a system message displayed, but there's none)