I want to change the user configurable timezone format. For example now it displaying as Asia/Kolkata: Friday, October 9, 2015 - 17:24 +0530 I want to display it as Asia/Kolkata. I have used following code but not working.

function my_modue_form_system_regional_settings_alter(&$form, &$form_state, $form_id) {
dpm(timezone_identifiers_list());
dpm($form);
$form['timezone']['date_default_timezone']['#options'] = timezone_identifiers_list();
}

Is it possible to do?