DateTime operator- (const DateTime& first, const Duration& second) { return first - second.YearMonth() - second.DayTime(); }
bool operator== (const Duration& first, const Duration& second) { return first.YearMonth() == second.YearMonth() && first.DayTime() == second.DayTime(); }
DateTime operator+ (const DateTime& first, const Duration& second) { // order is important: return first + second.YearMonth() + second.DayTime(); }