Form builder submit handler; Handles submission for regional settings.
function system_regional_settings_submit($form, &$form_state) {
config('system.date')
->set('country.default', $form_state['values']['site_default_country'])
->set('first_day', $form_state['values']['date_first_day'])
->set('timezone.default', $form_state['values']['date_default_timezone'])
->set('timezone.user.configurable', $form_state['values']['configurable_timezones'])
->set('timezone.user.warn', $form_state['values']['empty_timezone_message'])
->set('timezone.user.default', $form_state['values']['user_default_timezone'])
->save();
}