C LNK2019未解析的外部符号

我已阅读很多关于LNK2019的帖子，但无法解决此错误 .

这是我的代码：

time.h中：

#ifndef PROJECT2_TIME_H
#define PROJECT2_TIME_H

#include<iostream>
using std::ostream;

namespace Project2  
{
class Time
{
    friend Time& operator+=(const Time& lhs, const Time& rhs);
    friend ostream& operator<<(ostream& os, const Time& rhs);
public:
    static const unsigned secondsInOneHour = 3600;
    static const unsigned secondsInOneMinute = 60;
    Time(unsigned hours, unsigned minutes, unsigned seconds);
    unsigned getTotalTimeAsSeconds() const;
private:
    unsigned seconds;
};

Time& operator+=(const Time& lhs, const Time& rhs);
ostream& operator<<(ostream& os, const Time& rhs);

}

#endif

Time.cpp：

#include "Time.h"


Project2::Time::Time(unsigned hours, unsigned minutes, unsigned seconds)   
{
    this->seconds = hours*secondsInOneHour + minutes*secondsInOneMinute + seconds;
}

unsigned
Project2::Time::getTotalTimeAsSeconds() const
{
return this->seconds;
}


Project2::Time&
Project2::operator+=(const Time& lhs, const Time& rhs)
{
Time& tempTime(unsigned hours, unsigned minutes, unsigned seconds);
unsigned lhsHours = lhs.seconds / Time::secondsInOneHour;
unsigned lhsMinutes = (lhs.seconds / 60) % 60;
unsigned lhsSeconds = (lhs.seconds / 60 / 60) % 60;
unsigned rhsHours = rhs.seconds / Time::secondsInOneHour;
unsigned rhsMinutes = (rhs.seconds / 60) % 60;
unsigned rhsSeconds = (rhs.seconds / 60 / 60) % 60;
return tempTime(lhsHours + rhsHours, lhsMinutes + rhsMinutes, lhsSeconds + rhsSeconds);
}

ostream&
Project2::operator<<(ostream& os, const Time& rhs)
{
unsigned rhsHours = rhs.seconds / Time::secondsInOneHour;
unsigned rhsMinutes = (rhs.seconds / 60) % 60;
unsigned rhsSeconds = (rhs.seconds / 60 / 60) % 60;
os << rhsHours << "h:" << rhsMinutes << "m:" << rhsSeconds << "s";
return os;
}

main.cpp只是简单地创建了Time对象并使用了重载的运算符，似乎没有问题（这些代码都是提供的，因此它们本身就很好） .

我试图删除所有“时间”符号后面的“＆”，我得到了同样的错误 .

And here is the error message:

错误1个错误LNK2019：解析外部符号 “类Project2的::时间和__cdecl tempTime（无符号整数，无符号整型，无符号整型）” 函数“类Project2的引用（tempTime @@ @ YAAAVTime Project2的@@ III @ Z）： ：时间＆__cdecl Project2的::运算=（类Project2的::时间常量＆，类Project2的::时间常量＆）”（?? YProject2 @@ @ YAAAVTime 0 @ ABV10 @ 0 @ Z）C：\用户\ Eon- Gwei \ documents \ visual studio 2013 \ Projects \ c III_Project2_GW \ c III_Project2_GW \ Time.obj c III_Project2_GW