输入一个日期，求是这一年中的第几天

#include<bits/stdc++.h>
using namespace std;
bool is_leap_year(int year)
{
  if((year%400==0)||((year%4==0)&&(year%100!=0)))
  {
    return true;
  }
  else
    return false;
}
int main()
{
  int year,month,date;
  int day;
  bool flag = false;
  int sum=0;
  do
  {
    flag=false;
    cout<<"input: year month date"<<endl;
    cin>>year>>month>>date;
    if(year<0)
    {
      cout<<"输入年份错误，请重新输入";
      flag=true;
    }
    if(month<1||month>12)
    {
      cout<<"输入月份错误，请重新输入";
      flag=true;
    }
    if(month==1||month==3||month==5||month==7||month==8||month==10||month==12)
    {
      if(date>31||date<1)
      {
        cout<<"日期输入有误，请重新输入"<<endl;
        flag=true;
      }
    }
    if(month==4||month==6||month==9||month==11)
    {
      if(date>30||date<1)
      {
        cout<<"日期输入有误，请重新输入"<<endl;
        flag=true;
      }
    }
    if(is_leap_year(year)&&month==2)
    {
      if(date>29||date<1)
      {
        cout<<"本月是闰月，请从1-29重新输入日期"<<endl;
        flag=true;
      }
    }
    if(!is_leap_year(year)&&month==2)
    {
      if(date>28||date<1)
      {
        cout<<"本月是二月，请从1-28重新输入日期"<<endl;
        flag=true;
      }
    }
  }
  while(flag);
  for(int i=1; i<month; i++)
  {
    switch(i)
    {
      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        day=31;
        sum+=day;
        break;
      case 4:
      case 6:
      case 9:
      case 11:
        day=30;
        sum+=day;
      case 2:
        if(is_leap_year(year))
        {
          day=29;
          sum=sum+day;
          break;
        }
        else
        {
          day=28;
          sum=sum+day;
          break;
        }
    }
  }
  sum+=date;
  cout<<year<<"年"<<month<<"月"<<date<<"日是这一年的第"<<sum<< "天."<<endl;
}