Определение разности времени

#include <iostream>
 
struct MyTime
{
    size_t h, m, s;
    MyTime(void) : h(24), m(60), s(60) {}
    bool isValid(void) const
    {
        return ((h > 23 || m > 59 || s > 59) ? false : true);
    }
};
 
std::istream & operator>>(std::istream &in, MyTime &t)
{
    std::cout << "ч(ХХ): ";
    in >> t.h;
    std::cout << "м(ХХ): ";
    in >> t.m;
    std::cout << "с(ХХ): ";
    in >> t.s;
    std::cout << std::endl;
    return in;
}
 
std::ostream & operator<<(std::ostream &out, const MyTime &t)
{
    out << "ч(ХХ): " << t.h << std::endl;
    out << "м(ХХ): " << t.m << std::endl;
    out << "с(ХХ): " << t.s << std::endl;
    out << std::endl;
    return out;
}
 
MyTime operator-(const MyTime &t1, const MyTime &t2)
{
    if (t1.h < t2.h || !t1.isValid() || !t2.isValid())
        return MyTime();
 
    MyTime ret;
    int tmp = t1.s - t2.s;
    tmp >= 0 ? (ret.s = tmp, ret.m = 0) : (ret.s = 60 - t2.s + t1.s, ret.m = -1);
    tmp = t1.m - t2.m;
    tmp >= 0 ? (ret.m += tmp, ret.h = 0) : (ret.m += 60 - t2.m + t1.m, ret.h = -1);
    ret.h += t1.h - t2.h;
    
    return ret;
}
 
int main()
{
    setlocale(LC_ALL, "");
    MyTime t1, t2;
    std::cin >> t1 >> t2;
    std::cout << t2 - t1;
    return 0;
}