紧急求助：关于CommonsNet包，为什么调用FTPFile.setTimestamp()不能修改服务器端的文件上次修改时间啊？

Question

1。 为什么调用FTPFile.setTimestamp()不能修改服务器端的文件上次修改时间啊？                if (loginOK) {
                    System.out.println("Logged to FTP server.");
                    ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
                    ftpClient.enterLocalPassiveMode();
                    FTPFile[] ftpFiles = ftpClient.listFiles();
                    int ftpFilesLength = ftpFiles.length;
                    boolean found = false;
                    int i = 0;
                    for (; i < ftpFilesLength; i++) {
                        if (ftpFiles[i].getName().compareTo("Anlage_InfoCard.doc") == 0) {
                            System.out.println("old timestamp: " + ftpFiles[i].getTimestamp().getTime());
                            found = true;
                            break;
                        }
                    }
                    if (found) {
                        Date now = new Date();
                        Calendar CNow = Calendar.getInstance();
                        CNow.setTime(now);
                        ftpFiles[i].setTimestamp(CNow);
                        System.out.println("Now: " + now);
                        System.out.println("new timestamp of Anlage_InfoCard.doc: " + ftpFiles[i].getTimestamp().getTime());
                    }
                    ftpClient.logout();
                    ftpClient.disconnect();
上面代码执行完毕后察看，时间没有发生任何变化。
2。 如何才可以同步ftp服务器上的文件上次修改时间和本地同名文件的修改时间啊？？