为什么这个万年历的程序不能显示某年某月有的天数

这个程序为什么显示不了某年某月有的天数，，选择2不显示，请大家帮忙。

#include<iostream.h>
#include<stdlib.h>
#include<stdio.h>
#include<iomanip.h>
struct mon
{
  int maxdata;
  int data;
};
int SeekWeekDay(void);
int WeekDay(int year,int month,int day);
int HowManyDays(void);
int MonthDays(int year,int month);
int PrintWeek(int weekday);
int PrintMonth(int month);
int PrintData(void);
int main()
{
char choice;
while(1)
{
     puts("*******************************");
     puts("*请输入您的选择：            *");
     puts("*输入1求某个日期对应的星期   *");
     puts("*输入2求某年某月有的天数     *");
     puts("*输入3输出某年的日历         *");
     puts("*输入Q结束程序               *");
     puts("*******************************");
     cin>>choice;
     fflush(stdin);
     switch(choice)
     {
         case '1':SeekWeekDay(); break;
         case '2':int HowManyDays(int) ; break;
         case '3':PrintData(); break;
         case 'Q':return 0;
         default: puts("输入错误，请重新输入");break;
     }
    cout<<endl<<endl;
}
system("pause");
return 0;
}
int HowWanyDays(int)
{
    int year,month,days;
    cout<<"请输入年号和月份:";
    cin>>year>>month;fflush(stdin);
    cout<<"你的输入为"<<year<<"年"<<month<<"月,";
    days=MonthDays(year,month);
    cout<<year<<"年"<<month<<"月有"<<days<<"天\n";
    return 0;
}
SeekWeekDay()
{
    int year,month,day,weekday;
    cout<<"请输入年 月 日:";
    cin>>year>>month>>day; fflush(stdin);
    cout<<"你的输入为"<<year<<"年"<<month<<"月"<<day<<"日\n";
    weekday=WeekDay(year,month,day);
    cout<<"这天是" ;
    //int PrintWeek(int weekday);
    PrintWeek(weekday);
    return 0;
}
  PrintWeek(int weekday)
{
    switch(weekday)
    {
    case 0:cout<<"周日";break;
    case 1:cout<<"周一";break;
    case 2:cout<<"周二";break;
    case 3:cout<<"周三";break;
    case 4:cout<<"周四";break;
    case 5:cout<<"周五";break;
    case 6:cout<<"周六";break;
    }
    return 0;
}
PrintMonth(int month)
{
    switch(month)
    {
            case 1:cout<<"一  月"; break;
            case 2:cout<<"二  月"; break;
            case 3:cout<<"三  月"; break;
            case 4:cout<<"四  月"; break;
            case 5:cout<<"五  月"; break;
            case 6:cout<<"六  月"; break;
            case 7:cout<<"七  月"; break;
            case 8:cout<<"八  月"; break;
            case 9:cout<<"九  月"; break;
            case 10:cout<<"十  月";break;
            case 11:cout<<"十一 月";break;
            case 12:cout<<"十二 月";break;
                }
    return 0;
}
int WeekDay(int year,int month,int day)
{
    int i;
    int run=0,ping=0;
    long sum;
    for(i=1;i<year;i++)
    {
        if(i%4==0 && i%100!=0||i%400==0)
            run++;
        else
            ping++;
    }
    sum=366*run+365*ping;
    for(i=1;i<month;i++)
        sum+=MonthDays(year,i);
    sum+=day;
    return(int)sum&7;
}
int MonthDays(int year,int month)
{
    switch(month)
    {
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 10:
    case 12: return 31;
    case 4:
    case 6:
    case 9:
    case 11: return 30;
    case 2:
        if(year%4==0 && year&100!=0|| year%400==0)
            return 29;
        else
            return 28;
    default:cout<<"这是一个错误的月份!";system("pause");
        return 0;
    }
}
int PrintData(void)
{
    struct mon month[13];
    int i,j,k;
    int year,mon,week;
    cout<<"请输入年号" ;
cin>>year;
for(i=1;i<13;i++)
{
  month[i].data=1;
  month[i].maxdata=MonthDays(year,i);
}
for(i=0;i<6;i++)
{
  for(j=1;j<=2;j++)
   {
     mon=2*i+j;
     cout<<setw(20);
     PrintMonth(mon);
     cout<<setw(20);
     if(j==1)
       cout<<'\t';
  }
cout<<endl;cout<<endl;
for(j=1;j<=2;j++)
{
  for(k=0;k<7;k++)
  {
PrintWeek(k);
  }
cout<<setw(3)<<' ';
}
cout<<endl;
for(j=1;j<=2;j++)
{
   mon=2*i+j;
week=WeekDay(year,mon,1);
if( j==1)
cout<<setw(week*5+5)<<month[mon].data++;
else
cout<<setw(week*4.8)<<month[mon].data++;
week++;
while(week<7)
{
cout<<setw(5)<<month[mon].data++;
week++;
}
if(j==1)
cout<<setw(11)<<' ';
}
cout<<endl;cout<<endl;
while(month[2*i+1].data<=month[2*i+1].maxdata||month[2*i+2].data<=month[2*i+2].maxdata)
{
for(j=1;j<=2;j++)
  {
mon=2*i+j;
for(k=0;k<7;k++)
{
  if(month[mon].data<=month[mon].maxdata)
    cout<<setw(5)<<month[mon].data++;
    else
     cout<<"    ";
  }
if(j==1)
cout<<'\t';
}
cout<<endl;cout<<endl;
   }
  cout<<endl;
}
return 0;
}