@Override
    public void activateOptions() {
        super.activateOptions();
        if (datePattern != null && fileName != null) {
            now.setTime(System.currentTimeMillis());
            sdf = new SimpleDateFormat(datePattern);
            int type = computeCheckPeriod();
            printPeriodicity(type);
            rc.setType(type);
            File file = new File(fileName);
            scheduledFilename = fileName + sdf.format(new Date(file.lastModified()));
        }
        else {
            LogLog.error("Either File or DatePattern options are not set for appender [" + name + "].");
        }
    }
Returns the value of the <b>DatePattern</b> option.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/DailyRollingFileStreamAppender.java