Menu loader function for a date format.
$date_format_id: The date type name.
The date format id if it exists and is not locked, FALSE otherwise.
function system_date_format_load($date_format_id) {
if (($format = system_get_date_formats($date_format_id)) && empty($format['locked'])) {
return $date_format_id;
}
return FALSE;
}