Invalid < operator

Hy! This is my first question on this site. Sorry for my English, if I will make mistakes:(

So, my problem is the following. I have simple class Date.

class Date
{
public:
    Date();
    Date(unsigned short day, unsigned short month, unsigned short year);
    Date(const Date &date);
    unsigned short getDay();
    unsigned short getMonth();
    unsigned short getYear();

    void setDay(unsigned short day);
    void setMonth(unsigned short month);
    void setYear(unsigned short year);

void printOnScreen()const;
friend
    std::ostream& operator<< (std::ostream& out, const Date& date) {
        out << date.day << "." << date.month << "." << date.year;
        return out;
    }

friend
    bool operator<(const Date& a, const Date& b) {
        if (a == b) {
            return false;
        }

        if (a.year < b.year) {
            return true;
        }
        if (a.month < b.month) {
            return true;
        }
        if (a.day < b.day) {
            return true;
        }

        return false;
}

friend
    Date& operator-(Date& a) {
        return a;
    }

friend
    Date operator-(const Date& a, const Date& b) {
        return Date(
            abs(a.day - b.day),
            abs(a.month - b.month),
            abs(a.year - b.year)
            );
    }

friend
    bool operator==(const Date& date1, const Date& date2) {

        return (
            date1.day == date2.day &&
            date1.month == date2.month &&
            date1.year == date2.year
            );
    }

    virtual ~Date();
private:
    friend KeyHasher;

    unsigned short day;
    unsigned short month;
    unsigned short year;
};

In my main function I call sort like in this example, and after it get the error.

auto dates = {
    Date(1, 5, 2016),
    Date(3, 2, 2015),
    Date(3, 3, 2000),
    Date(2, 1, 1991),
    Date(1, 8, 2200),
    Date(1, 8, 2200),
    Date(1, 8, 2020),
    Date(21, 9, 2016)
};

vector<Date> v1(dates);
sort(
    v1.begin(),
    v1.end(),
    less<Date>()
);

What is wrong, I don't understand. Thank you for help.