The built-in + operator is a binary operator that takes two arguments of the same type and returns the sum of its arguments without changing their values. In addition, + is a commutative operator. This means that you can swap the operands' positions and still get the same result. Likewise, an overloaded version of operator + should reflect all these characteristics.
When overloading +, you can either declare it as a member function of its class or as a friend function. For example:
Date operator +(const Date& other); //member function
friend Year operator+ (const Year y1, const Year y2); //friend
Year operator+ (const Year y1, const Year y2);
The friend version is preferred because it reflects symmetry between the two operands. Since built-in + does not modify any of its operands, the parameters of the overloaded + are declared const. Finally, overloaded + should return the result of its operation by value, not by reference.