How can I add something after the day/hour/time in a date select list?
You can't use hook_form_alter() or hook_field_widget_form_alter() in this case. Try with hook_element_info_alter()

function MY_MODULE_element_info_alter(array &amp;$types) {
  if (isset($types['datelist'])) {
     $types['datelist']['#process'][] = '_mymodule_add_suffix_value';
  }
}

function _mymodule_add_suffix_value(&amp;$element) {
  // process logic option $element['day']
  // process $element['month']
  // process $element['hour']
  return $element;
}