计算两个日期之间相差几天

*************************Day.h***************************
#ifndef DAY_H
#define DAY_H
#include<iostream>
using namespace std;
class Day
{private:
int year;
int month;
int day;
public:
Day();
Day(int,int,int);
void input();
void Daysub(Day D1,Day D2);
void subdisplay();
void display();
};
#endif
**************************DayDef.cpp*************************
#include"Day.h"
Day::Day()
{}
Day::Day(int y,int m,int d)
{year=y;
month=m;
day=d;
}
void Day::input()
{cout<<"年：";
cin>>year;
cout<<"月：";
cin>>month;
cout<<"日：";
cin>>day;
}
void Day::Daysub(Day d1,Day d2)
{int i,c1=0,c2=0,t1,t2;
int m[13]={31,28,31,30,31,30,31,31,30,31,30,31};
for(i=1;i<=d1.year;i++)
if ((i% 4 ==