public static function getDayTimestamp($now = null, $offset = 8) {
        $now = $now === null ? time() : $now;
        
        $gtm0 = $now - ($now % 86400); // GTM 0
        
        $dayline = ($now - $gtm0) / 3600;
        
        // 时差跨天
        if ($dayline + $offset > 23) {
            return $gtm0 + 86400 + $offset * 3600 * - 1;
        } else {
            return $gtm0 + $offset * 3600 * - 1;
        }
    }