Ajax callback; Returns the date for a given format string.
function system_date_time_lookup($form, &$form_state) {
$format = '';
if (!empty($form_state['values']['date_format_pattern'])) {
$format = t('Displayed as %date_format', array(
'%date_format' => format_date(REQUEST_TIME, 'custom', $form_state['values']['date_format_pattern']),
));
}
// Return a command instead of a string, since the Ajax framework
// automatically prepends an additional empty DIV element for a string, which
// breaks the layout.
$response = new AjaxResponse();
$response
->addCommand(new ReplaceCommand('#edit-date-format-suffix', '<small id="edit-date-format-suffix">' . $format . '</small>'));
return $response;
}