How Friend function works

I have little problem but i don't know how to fix it. I wanted to test the friend function for the first time but I got error:

|17|error: 'minutes' was not declared in this scope|
|18|error: 'hours' was not declared in this scope|
|24|error: 'minutes' was not declared in this scope|
|24|error: 'minutes' was not declared in this scope|

That is my all code:

#include <iostream>
using namespace std;

class Time
{

    int hours;
    int minutes;
    friend Time operator+(const Time & t);
    friend void x(Time h, Time m );

};

Time operator+(const Time & t)
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}


void x(Time h, Time m) {hours = h; minutes = m;}