อธิบายการทำงานของโปรแกรมหน่อยครับผมยังไม่ค่อยเข้าใจ ภาษาC# Win (Windows App)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WindowsFormsApplication1
{
    class DateDifference
    {
        private DateTime _FromDate;
        private DateTime _ToDate;
        private int _Year = 0;
        private int _Month = 0;
        private int _Day = 0;

        public int Year
        {
            get { return _Year; }
            set { _Year = value; }
        }
        public int Month
        {
            get { return _Month; }
            set { _Month = value; }
        }
        public int Day
        {
            get { return _Day; }
            set { _Day = value; }
        }
        public DateTime FromDate
        {
            get { return _FromDate; }
            set { _FromDate = value; }
        }
        public DateTime ToDate
        {
            get { return _ToDate; }
            set { _ToDate = value; }
        }

        /// <summary>
        /// defining Number of days in month; index 0=> january and 11=> December
        /// february contain either 28 or 29 days, that's why here value is -1
        /// which wil be calculate later.
        /// </summary>
        private int[] monthDay = new int[12] { 31, -1, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

        public DateDifference(DateTime argFromDate)
        {
            _ToDate = DateTime.Now;
            _FromDate = argFromDate;
            CalcDifferenceDate();
        }

        public DateDifference(DateTime argFromDate, DateTime argToDate)
        {
            _ToDate = argToDate;
            _FromDate = argFromDate;
            CalcDifferenceDate();
        }

        private void SwapDate(ref DateTime LeftDate, ref DateTime RightDate)
        {
            DateTime tempDate;
            if (LeftDate > RightDate)
            {
                tempDate = LeftDate;
                LeftDate = RightDate;
                RightDate = tempDate;
            }
            else
            {
                LeftDate = LeftDate;
                RightDate = RightDate;
            }
        }

        private void CalcDifferenceDate()
        {

            SwapDate(ref _FromDate, ref _ToDate);
            int carryFlag = 0;

            // ***************************
            // Day calculation 
            if (this.FromDate.Day > this.ToDate.Day)
                carryFlag = this.monthDay[this.FromDate.Month - 1];

            // febuary detect
            if (carryFlag == -1)
            {
                if (CheckedLeapYear(this.FromDate))
                    // leap year february contain 29 days
                    carryFlag = 29;
                else
                    carryFlag = 28;
            }

            if (carryFlag != 0)
            {
                this.Day = (this.ToDate.Day + carryFlag) - this.FromDate.Day;
                carryFlag = 1;
            }
            else
                this.Day = this.ToDate.Day - this.FromDate.Day;

            // ***************************
            // Month calculation 

            if ((this.FromDate.Month + carryFlag) > this.ToDate.Month)
            {
                this.Month = (this.ToDate.Month + 12) - (this.FromDate.Month + carryFlag);
                carryFlag = 1;
            }
            else
            {
                this.Month = this.ToDate.Month - (this.FromDate.Month + carryFlag);
                carryFlag = 0;
            }

            this.Year = this.ToDate.Year - (this.FromDate.Year + carryFlag);
        } 


        private bool CheckedLeapYear(DateTime checkedDate)
        {
            int myYear = checkedDate.Year;
            return (((myYear % 4) == 0) && ((myYear % 100) != 0) || ((myYear % 400) == 0));
        }

        public string ToString(string argYearUnit, string argMonthUnit, string argDayUnit)
        {
            string retStr = string.Empty;
            if (this.Year > 0)
                retStr = retStr + string.Format("{0} {1} ", this.Year.ToString("#,##0"), argYearUnit);
            if (this.Month > 0)
                retStr = retStr + string.Format("{0} {1} ", this.Month.ToString("#,##0"), argMonthUnit);
            if (this.Day > 0)
                retStr = retStr + string.Format("{0} {1} ", this.Day.ToString("#,##0"), argDayUnit);
            return retStr.Trim();
        }

        public override string ToString()
        {
            return this.ToString("ปี ", "เดือน ", "วัน ");

        }
    }
}