计算任意一天是星期几

#include<iostream>
#include<string>
using namespace std;
int main()
{
char *p1=new char[50],*p2=new char[50];
int Y,M,D;
long int W;
gets(p1);
sprintf(p2,"%08s",p1);//将输入的年月规格化为8位，在左边补零。
sscanf(p2,"%04d",&Y);//提取年份
sscanf(p2+4,"%02d",&M);//提取月份
sscanf(p2+6,"%02d",&D);//提取天
W=(Y-1)+(Y-1)/4-(Y-1)/100+(Y-1)/400+D;
for(int i=1;i<M;i++)
{
switch(i){
case 1:
W+=31;break;
case 2:
W+=28;break;
case 3:
W+=31;break;
case 4:
W+=30;break;
case 5:
W+=31;break;
case 6:
W+=30;break;
case 7:
W+=31;break;
case 8:
W+=31;break;
case 9:
W+=30;break;
case 10:
W+=31;break;
case 11:
W+=30;break;
case 12:
W+=31;break;
}

}
if((((Y%4)==0)&&((Y%100)!=0))||(Y%400==0))
W+=1;
cout<<W%7<<endl;
}