日期計算函數：已知當前日期，求過了一段時間後的日期。

函數的功能是，根據當前日期，計算過了一段時間後的日期。

1. 流程圖：

2.函數實現：

    /**
      * Method called to check if the date is valid. 
      * short duration: unit is day. 
      * short[] curData: current date, format is YYMMDD
      * byte [] expData: store the expected date, format is YYMMDD
      * Return value: false if fail, else succeed.
      */
    private static boolean vGetExpectDate(short[] curDate, short duration, short [] expDate)
    {
        short year;
        short month;
        short day;
        short days_left;
        short day_add = 0;

        // Assign the real date to year, month, day and hour
        year = curDate[0];
        month = curDate[1];
        day = curDate[2];

        day_add = duration;

        // days add to year
        while(day_add > (days_left = sDaysToNextYear(year, month, day)))
        {
            // come to the 1st day in 1st month of the next year.
            year++;
            month = 1;
            day = 1;
            day_add -= days_left;
        }


        // days add to month
        while(day_add > (days_left = sDaysToNextMonth(year, month, day)))
        {
            month++;
            day = 1;
            day_add -= days_left;
        }

        // days add to day
        day += day_add;

        // store the expected date into the workbuf.
        expDate[0] = year;
        expDate[1] = month;
        expDate[2] = day;

        return true;
    }


    private static short sDaysToNextYear(byte year, byte month, byte day)
    {
        byte days_Feb;
        byte days_month;
        short ret;

        switch(month)
        {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                days_month = 31;
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                days_month = 30;
                break;
            case 2:
                // Here assumes the year is 20xx, so if the year is leap or not depends on the last byte xx.
                if(0 == (year % 4))
                    days_month = 29;
                else
                    days_month = 28;
                break;
            default:
                break;
        }

        ret = days_month - day + 1;
        month++;

        switch(month)
        {
            case 2:
                ret += days_Feb;
            case 3:
                ret += 31;
            case 4:
                ret += 30;
            case 5:
                ret += 31;
            case 6:
                ret += 30;
            case 7:
                ret += 31;
            case 8:
                ret += 31;
            case 9:
                ret += 30;
            case 10:
                ret += 31;
            case 11:
                ret += 30;
            case 12:
                ret += 31;
            default:
                break;
        }

        return ret;
    }

    private static short sDaysToNextMonth(byte year, byte month, byte day)
    {
        byte days_month;

        switch(month)
        {
            case 1:
                days_month = 31;
                break;
            case 2:
                // Here assumes the year is 20xx, so if the year is leap or not depends on the last byte xx.
                if(0 == (year % 4))
                    days_month = 29;
                else
                    days_month = 28;
                break;
            case 3:
                days_month = 31;
                break;
            case 4:
                days_month = 30;
                break;
            case 5:
                days_month = 31;
                break;
            case 6:
                days_month = 30;
                break;
            case 7:
                days_month = 31;
                break;
            case 8:
                days_month = 31;
                break;
            case 9:
                days_month = 30;
                break;
            case 10:
                days_month = 31;
                break;
            case 11:
                days_month = 30;
                break;
            case 12:
                days_month = 31;
                break;
            default:
                break;
        }

        return (days_month - day + 1);
    }