修改以下程序，如何用sub函数完成减法运算

#include
using namespace std;
class Time
{
public:
Time(int nHours = 0, int nMins = 0); // 构造函数
Time operator-(Time time2);
// 运算符“+”重载为成员函数
int GetHours() { return m_nHours; } // 获取小时数
int GetMins() { return m_nMins; } // 获取分钟数
void Show(); // 显示时间值
private:
int m_nHours, m_nMins; // 小时数&分钟数
};
Time::Time(int nHours, int nMins)
// 构造函数的实现
{
nHours += nMins / 60;
nMins %= 60;
m_nHours = nHours;
m_nMins = nMins;
}
void Time::Show()
{
cout << m_nHours << "小时" << m_nMins << "分钟" << endl;
}
Time Time::operator-(Time time2)
// 重载运算符函数实现
{
int nNewHours, nNewMins, nhour2, nmin2;
nmin2 = time2.GetMins();
nhour2 = time2.GetHours();
if (nmin2 > m_nMins)
{
nNewMins = m_nMins - nmin2 + 60;
m_nHours--;
}
else
nNewMins = m_nMins - nmin2;
nNewHours = m_nHours - nhour2;
return Time(nNewHours, nNewMins);
}
int main()
{
Time time1(18, 40);
Time time2(21, 50);
Time timeSub;
cout << "time1: ";
time1.Show();
cout << "time2: ";
time2.Show();
timeSub = time2 - time1;
cout << "time2 – time1 : ";
timeSub.Show();
return 0;
}