Drupal 8 calculate separate time for date time field

mymodule.php

<?php

/**
 * Implements hook_preprocess_HOOK().
 */
function module_preprocess_node__content_type(&$variables) {
  $node = $variables['elements']['#node'];

  // Get time as separate variable.
  // Calculate using system timezone.
  if (!$node->field_my_datetime->isEmpty()) {
    $date_string = $node->field_my_datetime->value;
    
    $timezone = \Drupal::config('system.date')->get('timezone.default');
    
    $mydate = new DateTime($date_string, new DateTimeZone('UTC'));
    $mydate->setTimezone(new DateTimeZone($timezone));
    
    $variables['time_only'] = $mydate->format('H:i');
  }
}