$obCache = new CPHPCache;
$life_time = 86400;
$cache_id = date("dmY") . "cities";

// если кеш есть и он ещё не истек то
if ($obCache->InitCache($life_time, $cache_id, "/" . SITE_ID . "/custom/")) {
    $vars = $obCache->GetVars();
    $arResult["ITEMS"] = $vars["ITEMS"];
} else {
    CModule::IncludeModule("iblock");
    $ib_cities = 12;
    $ib_courses = 13;
    $ib_schedule = 14;

    $arSelect = Array("NAME");
    $arFilter = array(
        "IBLOCK_ID" => $ib_courses,
        "ID" => CIBlockElement::SubQuery("PROPERTY_COURSE_ID", Array(
            "IBLOCK_ID" => $ib_schedule,
            "ACTIVE" => "Y",
            ">=DATE_ACTIVE_FROM" => array(false, ConvertTimeStamp(false, "FULL")),
        )),
        "PROPERTY_CITY.NAME" => "Торжок"
    );
    $res = CIBlockElement::GetList(Array(), $arFilter, false, false, $arSelect);

    while ($fields = $res->GetNext()) {
        $arResult["ITEMS"][] = $fields;
    }

}

if ($obCache->StartDataCache()):
    $obCache->EndDataCache(
        array(
            "ITEMS" => $arResult["ITEMS"],
        )
    );
endif;