public long getNextCheckMillis(Date now) {
        return getNextCheckDate(now).getTime();
    }
RollingCalendar is a helper class to DailyRollingFileAppender. Given a
periodicity type and the current time, it computes the start of the next
interval.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/DailyRollingFileStreamAppender.java