on a comment the compact-user viewmode have a "member for x months" that is hardcoded & gets outputted as a form item, which looks like some old legacy code we never came around to clean up.
besides of that it have hardcoded markup & we dont like that
print it all to a template so the themer can work with it
From user.module's user_user_view():
$build['member_for'] = array( '#type' => 'item', '#markup' => '<h4 class="label">' . t('Member for') . '</h4> ' . \Drupal::service('date.formatter')->formatTimeDiffSince($account->getCreatedTime()), );