为什么我定义声明了add（）函数，可执行程序的时候程序提示add（）函数未定义呢？求解释，有点晕，想不出原因啊，拜托大家

#include<iostream>
using namespace std;
class Time
{
      private:
      int hours,minutes,seconds;
      public:
      Time(void){};
      Time(int h,int m,int s);
      Time(Time &t);
      ~Time(){cout<<"the end"<<endl;}
      void settime();
      int  gettime();
      void add(Time&t1,Time&t2);
};
Time::Time(int h,int m,int s)
{
      hours=h;minutes=m;seconds=s;
      cout<<"现在的时间为："<<hours<<":"<<minutes<<":"<<seconds<<endl;
}
Time::Time(Time &t)
{
      hours=t.hours;minutes=t.minutes;seconds=t.seconds;
      cout<<"现在的时间为："<<hours<<":"<<minutes<<":"<<seconds<<endl;
}
void Time::settime()
{
     int h,m,s;
     cin>>h>>m>>s;
     hours=h;
     minutes=m;
     seconds=s;
}
int Time::gettime()
{
   cout<<"现在的时间为："<<hours<<":"<<minutes<<":"<<seconds<<endl;
   return 0;
}
void Time::add(Time&t1,Time&t2)
{
     Time total;
     const int perminutes=60;
     const int perhours=60;
     total.seconds=(t1.seconds+t2.seconds)%perminutes;
     total.minutes=(t1.minutes+t2.minutes+(t1.seconds+t2.seconds)/perminutes)%perhours;
     total.hours=(t1.hours+t2.hours+(t1.minutes+t2.minutes+(t1.seconds+t2.seconds)/perminutes)/perhours);
     cout<<"现在的时间为："<<total.hours<<":"<<total.minutes<<":"<<total.seconds<<endl;
     
}
int main()
{
    Time a(5,5,23);
    Time total;
    cout<<"请设置当前的时间："<<endl;
    a.settime();
    a.gettime();
    cout<<endl<<endl;
    Time b(6,4,34);
    total.add( a,b);         //error C2065: 'add' : undeclared identifier（成员函数的调用前面要写上对象名，形如:对象名.成员函数（参数),add是Time的成员函数,所以,要先定义另一个对像，然后再调用add函数。）
    system("pause");
    return 0;
}