Go: package main

import (
	"bufio"
		"os"
	"strings"
	"strconv"
	"time"
	"fmt"
)

var scanner = bufio.NewScanner(os.Stdin)

func main() {
	for scanner.Scan() {
		input := strings.Split(scanner.Text(), " ")
		ele_t1 := [3]int{}
		ele_t2 := [3]int{}
		for i := range ele_t1 {
			ele_t1[i], _ = strconv.Atoi(input[i])
			ele_t2[i], _ = strconv.Atoi(input[i+3])
		}
		if ele_t1[0] < 0 || ele_t1[1] < 0 || ele_t1[2] < 0 ||
			ele_t2[0] < 0 || ele_t2[1] < 0 || ele_t2[2] < 0 {
				break
		}
		t1 := time.Date(ele_t1[0], time.Month(ele_t1[1]), ele_t1[2], 0, 0, 0, 0, time.UTC)
		t2 := time.Date(ele_t2[0], time.Month(ele_t2[1]), ele_t2[2], 0, 0, 0, 0, time.UTC)
		diff := t2.Sub(t1)
		fmt.Printf("%v\n", int64(diff/(time.Hour*24)))
	}
}

C++: #include <iostream>
#include <sstream>


using namespace std;


int day_per_month[13] ={0,31, 28, 31, 30, 31, 30 ,31, 31, 30, 31, 30, 31};
int day_per_month_leap_year[13] ={0,31, 29, 31, 30, 31, 30 ,31, 31, 30, 31, 30, 31};

int main(){

    int y1, m1, d1, y2, m2, d2;
    while(cin>>y1>>m1>>d1>>y2>>m2>>d2){
        if(y1<0||m1<0||d1<0||y2<0||m2<0||d2<0){
            break;
        }
        int y,m;
        y=y1;
        int day = 0;

        
        while(1){
            if(y+1>y2)break;
            
            day+=365;
            if(y%4==0){
                day++;
                if(y%100==0){
                    day--;
                    if(y%400==0){
                        day++;
                    }
                }
            }
            y++;
            
        }
        
        if(m1<m2){
            m=m1;
            while(1){
                if(m+1>m2)break;
                double flag=false;
                if(y1%4==0){
                    flag=true;
                    if(y1%100==0){
                        flag=false;
                        if(y1%400==0){
                            flag=true;
                        }
                    }
                }
                
                if(flag){
                    day+=day_per_month_leap_year[m];
                    m++;
                }else{
                    day+=day_per_month[m];
                    m++;
                }
            }
        }else if(m1>m2){
            m=m2;
            while(1){
                if(m-1<m1)break;
                double flag=false;
                if(y1%4==0){
                    flag=true;
                    if(y1%100==0){
                        flag=false;
                        if(y1
                           %400==0){
                            flag=true;
                        }
                    }
                }
                
                if(flag){
                    m--;
                    day-=day_per_month_leap_year[m];
                }else{
                    m--;
                    day-=day_per_month[m];
                }
            }
        }
        //cout<<day<<endl;
        
        day+=d2-d1;
        
        cout<<day<<endl;
        
        
        
    }
}

C++: #include <bits/stdc++.h>
using namespace std;
const int month[] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };

bool is_leap(int y) {
  if ((y % 4 == 0) && ((y % 100 != 0) || (y % 400 == 0))) return true;
  else return false;
}

int get_sum(int y, int m, int d) {
  int sum = 0;
  for (int i = 0; i < y; i++) {
    if (is_leap(i)) sum += 366;
    else sum += 365;
  }
  for (int i = 0; i < m; i++) {
    if (i == 2 && is_leap(y)) sum += 29;
    else sum += month[i];
  }
  sum += d;
  return sum;
}

int main() {
  int y1, m1, d1, y2, m2, d2;
  while (cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2) {
    if (y1 < 0 || m1 < 0 || d1 < 0 || y2 < 0 || m2 < 0 || d2 < 0) break;
    cout << get_sum(y2, m2, d2) - get_sum(y1, m1, d1) << "\n";
  }
}

C++: #include<bits/stdc++.h>
using namespace std;

#define pb push_back
#define fr first
#define sc second
typedef vector<int> vi;
typedef pair<int,int> pii;
typedef pair<int,pii> pip; 
const int INF = (1<<25);
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};

int main(){
  int y1,m1,d1,y2,m2,d2;
  while(cin>>y1 >>m1 >>d1 >>y2 >>m2 >>d2){
    if(y1<0 || m1<0 ||  d1<0 ||  y2<0 || m2<0 ||  d2<0) break;
    long long int total1=0,total2=0;
    for(int i=0;i<y1;i++){
      if(i%400==0 || (i%100!=0 && i%4==0)){
	total1+=366;
	 
      }else {
	total1+=365;
      }
    }

    for(int i=0;i<y2;i++){
      if(i%400==0 || (i%100!=0 && i%4==0)){
	total2+=366;
	  
      }else {
	total2+=365;
      }
    }

    for(int i=1;i<m1;i++){
      if(i==2){
	if(y1%400==0 || ( y1%100!=0 || y1%4==0)){
	  total1+=29;
	  
	}
	else total1+=28;
      }
      else if(i==4 || i==6 || i==9 || i==11){
	total1+=30;
      }
      else total1+=31;
    }
    
    for(int i=1;i<m2;i++){
      if(i==2){
	if(y2%400==0 || ( y2%100!=0 && y2%4==0)){
	    total2+=29;
	  
	}
	else total2+=28;
      }
      else if(i==4 || i==6 || i==9 || i==11){
	total2+=30;
      }
      else total2+=31;
    }

    total1+=d1;
    total2+=d2;
    cout<< total2-total1<< endl;
  }
}