Ошибка: Левый операнд должен быть левосторонним значением

#include "stdafx.h"
#include <windows.h>
#include <iostream>
#include <clocale>
#include <stdlib.h>
#include <cstring>
#include <string.h>
using namespace std;
class Date
{
private:
    unsigned int day;
    unsigned int year;
    unsigned int mounth;
    int DayInMounth(int k);
    void DaysToDate(int _day);
    
public:
    int  DateToDays();
    void Init(int);
    void Init(int, int, int);
    void Init(char *str);
    Date FewDay(int);
    bool LeapYear();
    unsigned int GetDay();
    unsigned int GetYear();
    unsigned int GetMounth();
    void PutDay(unsigned int);
    void PutMounth(unsigned int);
    void PutYear(unsigned int);
    
};
void Date::DaysToDate(int _days)
{
    year = _days / 365;
    _days -= year * 365;
    mounth = 1;
    while (_days > DayInMounth(mounth))
    {
        _days -= DayInMounth(mounth);
        mounth++;
    }
    day = _days;
}
 
void Date::Init(int _day, int _mounth, int _year)
{
    year = _year;
    mounth = _mounth;
    if (mounth > 12)
        mounth = 12;
    day = _day;
    if (day > DayInMounth(mounth))
        day = DayInMounth(mounth);
}
 
void Date::Init(int _days)
{
    DaysToDate(_days);
}
 
void Date::Init(char *str)
{
    year = atoi(&str[0]);
    mounth = atoi(&str[5]);
    if (mounth > 12)
        mounth = 12;
    day = atoi(&str[8]);
    if (day > DayInMounth(mounth))
        day = DayInMounth(mounth);
}
 
unsigned int Date::GetDay()
{
    return day;
}
 
unsigned int Date::GetMounth()
{
    return mounth;
}
 
unsigned int Date::GetYear()
{
    return year;
}
 
int Date::DayInMounth(int k)
{
    switch (k)
    {
    case 1: return 31;
    case 2:
        if (LeapYear())
            return 29;
        else
            return 28;
    case 3: return 31;
    case 4: return 30;
    case 5: return 31;
    case 6: return 30;
    case 7: return 31;
    case 8: return 31;
    case 9: return 30;
    case 10:return 31;
    case 11:return 30;
    case 12:return 31;
    }
    return 0;
}
 
int Date::DateToDays()
{
    int result = day;
    for (unsigned int i = 1; i<mounth; i++)
        result += DayInMounth(i);
    result += year * 365;
    return result;
}
 
Date Date::FewDay(int _day)
{
    Date time_element;
    int res_day = DateToDays() + _day;
    time_element.Init(res_day);
    return time_element;
}
 
 
bool Date::LeapYear()
{
    bool flag = false;
    if (year % 4 == 0)
        flag = true;
    return flag;
}
 
void Date::PutDay(unsigned int _day)
{
    day = _day;
}
 
void Date::PutMounth(unsigned int _mounth)
{
    mounth = _mounth;
}
 
void Date::PutYear(unsigned int _year)
{
    year = _year;
}
 
class Goods
{
private:
    char *name;
    Date * date;
    double price;
    int count;
    int code;
public:
    Goods();
    void Read();
    void Display();
 
    void SetPrice();
    void Count(int value) {count += value;}
    double Price() { return count*price;}
    void Storage();
 
    char * ToString();
};
Goods::Goods()
{
    date = new Date();
    name = new char [255];
}
 
void Goods::Read()
{
    cout<<"Название : ";
    cin>>name;
    cout<<"Цена товара : ";
    cin>>price;
    cout<<"Номер накладной : ";
    cin>>code;
    cout<<"Кол-во : ";
    cin>>count;
    cout<<"Дата поступления  [gggg.mm.dd]: ";
    char *str = new char [20];
    cin>>str;
    date->Init(str);
}
 
void Goods::Display()
{
    cout<<"Название: "<<name<<endl;
    cout<<"Цена товара : "<<price<<endl;
    cout<<"Номер накладной: "<<code<<endl;
    cout<<"Кол-во : "<<count<<endl;
    cout<<"Дата оформления [gggg.mm.dd]: "<<date->GetYear()<<"."<<date->GetMounth()<<"."<<date->GetDay()<<endl;
}
 
void Goods::SetPrice()
{
    cout<<"Старая цена: "<<price<<endl;
    cout<<"Новая цена: ";
    cin>>price;
}
 
char * Goods::ToString()
{
    char * buf = new char [64];
    char * res = new char [64];
    double v = Price();
    int iv1 = int(v);
    v -= iv1;
    while(v - int(v) > 0.001)
        v *= 10;
    int iv2 = int(v);
 
    res[0] = '\0';
    itoa(iv1,buf,10);
    strcat(res,buf);
    strcat(res,new char = ",");
    itoa(iv2,buf,10);
    strcat(res,buf);
 
    return res;
}
 
void Goods::Storage()
{
    Date now;
    cout<<"Сейчас : "<<endl;
    cout<<"Дата [gggg.mm.dd]: ";
    char *str = new char [20];
    cin>>str;
    now.Init(str);
 
    cout<<now.DateToDays() - date->DateToDays()<<" days"<<endl;
}

strcat(res,",");