一个类的成员函数做为另一个类的友元函数

#include <iostream>
using namespace std;
class Date;
class Time
{public:
Time(int,int,int);
void display(const Date&);
private:
 int hour;
 int minute;
 int sec;
};

class Date
{public:
Date(int,int,int);
friend void Time::display(const Date &);
private:
 int month;
 int day;
 int year;
};

Time::Time(int h,int m,int s)
{hour=h;
minute=m;
sec=s;
}

void Time::display(const Date &da)
{cout<<da.month<<"/"<<da.day<<"/"<<da.year<<endl;
cout<<hour<<":"<<minute<<":"<<sec<<endl;
}

Date::Date(int m,int d,int y)
{month=m;
day=d;
year=y;
}

int main()
{Time t1(10,13,56);
Date d1(12,25,2004);
t1.display(d1);
return 0;
}