C++实现一个日期

#include<iostream>
using namespace std;
class Date
{
public:
 Date(int year = 1900, int month = 1, int day = 1)//类的初始化
  :_year(year)
  , _month(month)
  , _day(day)
 {
  if (!IsInvalidDate())
  {
   _year = 1900;
   _month = 1;
   _day = 1;
  }
 }
 Date operator+(int days)//实现日期的+操作

 {
  if (days<0)
   return  *this - (0 - days);
  Date temp(*this);
  temp._day += days;

  int DaysOfMonth = 0;
  while (temp._day >(DaysOfMonth = _CetDaysOfMonth(temp._year, temp._month)))
  {
   temp._day -= DaysOfMonth;
   ++temp._month;
   if (temp._month > 12)
   {
    temp._year += 1;
    temp._month = 1;
   }
  }
  return  temp;
 }


 Date operator-(int days)//实现日期的-操作
 {
  if (days < 0)
   return *this + (0 - days);
  Date temp(*this);
  temp._day -= days;
  while(temp._day <= 0)
  {
   temp._month -= 1;
   if (0 == temp._month)
   {
    temp._year -= 1;
    temp._month = 12;
   }
   temp._day += _CetDaysOfMonth(temp._year, temp._month);
  }
  return temp;
 }
 Date& operator++()//前置++
 {
  *this = *this + 1;
  return *this;
 }
 Date  operator++(int)//后置++
 {
  Date temp(*this);
  ++(*this);
  return temp;
 }
 Date& operator--()//前置--
 {
  *this = *this -1;
  return *this;
 }
 Date  operator--(int)//后置--
 {
  Date temp(*this);
  --(*this);
  return temp;
 }
 //2018 11 4
 //2019 1 1
 int operator-(const Date& d)
 {
  Date MinDate(*this);
  Date MaxDate(d);
  if (*this > d)
  {
   MinDate = d;
   MaxDate = *this;
  }
  int count = 0;
  while (MinDate != MaxDate)
  {
   ++count;
   ++MinDate;
  }
  return count;
 }
 bool operator==(const Date & d)const
 {
  return _year == d._year &&
   _month == d._month &&
   _day == d._day;
 }
 bool operator>(const Date& d)const
 {
  if ((_year > d._year) ||
   (_year == d._year && _month > d._month) ||
   (_year == d._year && _month == d._month && _day > d._day))
  {
   return true;
  }
  return false;
 }
 bool operator!=(const Date& d)const
 {
  return !(*this == d);
 } 
private:
 bool IsInvalidDate()//判断一个日期是否合法
 {
  if (((_year >0) &&
   (_month >0 && _month<13) &&
   (_day > 0 && _day <= _CetDaysOfMonth(_year, _month))))
  {
   return true;
  }
  return false;
 }
 int _CetDaysOfMonth(int year, int month)
 {
  int days[] = {0,31,28,31,30,31,30,31,31,30,31,30,31};
  if (2 == month && _IsLeapYear(year))
   days[month] += 1; 
  return days[month];
 }
 bool _IsLeapYear(int year)
 {
  if ((0 == year % 4 && 0 != year % 100) ||
   (0 == year % 400))
  {
   return true;
  }
  return false;
 }
 friend ostream& operator<<(ostream& _cout, const Date& d)
 {
  _cout << d._year << "-" << d._month << "-" << d._day;
  return _cout;
 }
 friend istream& operator>>(istream& _cin, Date& d)
 {
  _cin >> d._year >> d._month >> d._day; 
  if (!d.IsInvalidDate())
   cout << "输入的日期非法：" << d << endl;
  return _cin;
 }
private:
 int _year;
 int _month;
 int _day;
};
int main()
{
 Date d1(2018, 11, 4);
 cout << d1-999<< endl;
 Date d2(2019, 1, 1);
 cout << d1 - d2 << endl;
 cout << d2 - d1 << endl;
 return 0;
}