輸入年月日求這是今年多少天(for)

#include <iostream>
using namespace std;
int main()
{
 int t,n,y,d,m;
 int a=0;
 cout<<"enter the year as a number :";
 cin>>y;
 cout<<"enter the month of this year as a number:";
 cin>>m;
 cout<<"enter the day of this year as a number:";
 cin>>d;       //輸入年月日
    n=(y%4==0&&y%100!=0||y%100==0);   //判斷閏年
  if(m>12||m<1||d<1||d>31)
 {
  cout<<"it is not effort data:"<<endl;
  system("pause");
  return 0;
  
 }            
 if(m==2&&n==1&&d>29)
 {
         cout<<"it is not effort data:"<<endl;
      system("pause");
   return 0;
 }
 if(m==2&&n==0&&d>28)
 {
       cout<<"it is not effort data:"<<endl;
  system("pause");
  return 0;
 }          //判斷有效資料

 for(int i=1;i<m;i++)
 {
      t=31-((m==4)+(m==6)+(m==9)+(m==11))-((3-n)*(m==2));
   a+=t;
 }           //將月份累加
 a+=d;
 cout<<"this is the "<<a<<"  days of this year";
 system("pause");
 return 0;
}