C++ 判斷日期是第幾天

#include <iostream>
using namespace std;
struct Date{
	int year;
	int month;
	int day;
}date;
const int days[] = {31,28,31,30,31,30,31,31,30,31,30,31};
bool isLeapYear(int year){
	return year%4==0 && year%100!=0 || year%400==0;
}
int main(){
	cout<<"請依次輸入年月日(如 2020 01 01)：";
	cin>>date.year>>date.month>>date.day;
	int sum = 0;
	for(int i=0;i+1<date.month;i++) sum += days[i];
	sum += date.day;
	
	if(date.month == 2 && date.day >= 29 && !isLeapYear(date.year)) {
		cout<<"輸入有誤"<<endl;
		return 0; 
	}
	if(date.day>days[date.month-1]){
		cout<<"輸入有誤"<<endl;
		return 0; 
	}	
	if(date.month>2 && isLeapYear(date.year))sum++;
	cout<<"您輸入的是本年中的第"<<sum<<"天"; 
	return 0;
}