一个不错的代码复用

这是一个计算一年中的日期具体是一年的哪一天的小Date类，例如2010年10月1号是2010年的哪一天？感觉这个类中有些不错的代码亮点和思想。读一段代码，有时候真的是如读一个精彩的短篇小说一样的令人回味。

using System;
class Date
{
    int year;
    int month;
    int day;

    static int[] MonthDays = new int[] { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

    public Date()
    {
        Year = 1600;
        Month = 1;
        Day = 1;
    }

    public Date(int year, int month, int day)
    {
        if ((month == 2 && IsLeapYear(year) && day > 29) ||
            (month == 2 && !IsLeapYear(year) && day > 28) ||
            ((month == 4 || month == 6 || month == 9 || month == 11) && day > 30))
        {
            throw new ArgumentOutOfRangeException("Day");
        }
        else
        {
            Year = year;
            Month = month;
            Day = day;
        }
    }
    public int Year
    {
        set
        {
            if (value < 1600)
                throw new ArgumentOutOfRangeException("Year");
            else
                year = value;
        }
        get
        {
            return year;
        }
    }
    public int Month
    {
        set
        {
            if (value < 1 || value > 12)
                throw new ArgumentOutOfRangeException("Month");
            else
                month = value;
        }
        get
        {
            return month;
        }
    }
    public int Day
    {
        set
        {
            if (value < 1 || value > 31)
                throw new ArgumentOutOfRangeException("Day");
            else
                day = value;
        }
        get
        {
            return day;
        }
    }
    public static bool IsLeapYear(int year)
    {
        return (year % 4 == 0) && ((year % 100 != 0)) || (year % 400 == 0);
    }

    public int DayOfYear
    {
        get
        {
            return MonthDays[month - 1] + day + (month > 2 && IsLeapYear(year) ? 1 : 0);
        }
    }
}

下面是对这个类的一个应用，用于统计两个年份之间的具体相差天数

using System;

class CsDateInheritance
{
    public static void Main()
    {
        DatePlus birth = new DatePlus(1986, 11, 15);
        DatePlus today = new DatePlus(2010, 10, 12);

        Console.WriteLine("Birthday = {0}", birth);
        Console.WriteLine("Today = " + today);
        Console.WriteLine("Days since birthday = {0}", today - birth);
    }
}

class DatePlus : Date
{
    public DatePlus() { }
    public DatePlus(int year, int month, int day) : base(year, month, day) { }

    public int DaysSince1600
    {
        get
        {
            return 365 * (Year - 1600) + (Year - 1597) / 4 - (Year - 1601) / 100 + (Year - 1601) / 400 + DayOfYear;
        }
    }
    public override string ToString()
    {
        string[] str = { "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Ju", "Aug", "Sep", "Oct", "Nov", "Dec" };
        return String.Format("{0} {1} {2}", Day, str[Month - 1], Year);
    }

    public static int operator -(DatePlus date1, DatePlus date2)
    {
        return date1.DaysSince1600 - date2.DaysSince1600;
    }
}