关于两个类的友元函数的调用

#include <iostream>
using namespace std;
class Date;
class Time

 {public:
   friend void display(const Date &,const Time &);
   friend void get_value( Date &, Time &);
  private:
   int hour;
   int minute;
   int sec;

 };



class Date

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

 };

void display(const Date &d,const Time &t)

 {
  cout<<d.month<<"/"<<d.day<<"/"<<d.year<<endl;
  cout<<t.hour<<":"<<t.minute<<":"<<t.sec<<endl;

 }

void get_value(Date &d,Time &t)                                                  
{
    cout<<"please input date and time: ";
    cin>>d.month>>d.day>>d.year;                             
    cin>>t.hour>>t.minute>>t.sec;
}

int main()
{

 Date d1;

 Time t1;

 display(d1,t1);

 return 0;
}