Code Understanding Problem in C#

class DateClass
    {
        private int Day;
        private int Month;
        private int Year;
        public int year
        {
            get { return Year;}
            private set
            {
                if (Year < 0 || Year <= 1970 || Year >= 2051)
                {
                    throw new ArgumentOutOfRangeException(
                    "Year", Year, "Outof Range From the Current Program");
                }
                else
                {
                    Year = value;
                }
            }
        }
         public DateClass ( int D, int M, int Y )
        {
            Day = D;
            Month = M;
            Year = Y;
        }
        /// <summary>
        /// Set Month And Check The Condition
        /// </summary>
        public int SetMonth
        {
            get
            {
                return Month;
            }
            private set
            {
                if (Month > 0 && Month < 12)
                {
                    Month = value;
                }
                else
                {
                    throw new ArgumentOutOfRangeException(
                        "Month", value, "Month Must Be Between (1-12)");
                }
            }
        }
        public int SetDay
        {
            get
            {
                return Day;
            }
            private set
            {
                int[] DayArray={0,31,28,31,30,31,30,31,31,30,31,30,31};
                if (value > 0 && value <= DayArray[Month])
                {
                    Day = value;
                }
                else if (Month == 2 && Day == 29 && (Year % 400 == 0 || Year % 4 == 0 && year % 100 != 0))
                {
                    Day = value;
                }
                else
                    throw new ArgumentOutOfRangeException(
                        "Day ", Day, "day out of range of current month && Year");
            }
        }