Change default format for date field on user register form
You can implement template_preprocess_field() and alter the markup of the date field.

function MY_THEME_preprocess_field(&amp;$vars) {
  if ($vars['field_name'] === 'field_custom_date_field') {
    $date_format = 'short'; // You need to provide the machine name of the format you want.
    $timestamp = strtotime($vars['element']['#items']-&gt;value);
    $markup = \Drupal::service('date.formatter')-&gt;format($timestamp, $date_format);
    $vars['items'][0]['content']['#text'] = $markup;
  }
}