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>
using namespace std;
#define REP(i,b,n) for(int i=b;i<n;i++)
#define rep(i,n)   REP(i,0,n)

int month[13]={
  0,
  31,//1
  28,//2
  31,//3
  30,//4
  31,//5
  30,//6
  31,//7
  31,//8
  30,//9
  31,//10
  30,//11
  31//12
};

int day(int y,int m,int d){
  int base = (y-1)*365+d;
  REP(i,1,m)base+=month[i];
  int lim = m > 2 ? y+1:y;
  REP(i,1,lim){
    if ( i %400 == 0 ||(i % 100 !=0 && i % 4 == 0))base++;
  }
  return base;
}



/*
  correct version

int day2(int y,int m,int d){
  y += 4800;
  if (m < 3) --y, m += 12;
  return 365L*y+y/4-y/100+y/400+(153*m-457)/5+d-32045;
}
*/

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 a=day(y1,m1,d1),b=day(y2,m2,d2);
    
    
    
    cout << b-a << endl;
  }
}

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

#define pb(n) push_back(n)
#define fi first
#define se second
#define all(r) (r).begin(),(r).end()
#define gsort(st,en) sort((st),(en),greater<int>())
#define vmax(ary) *max_element(all(ary))
#define vmin(ary) *min_element(all(ary))
#define debug(x) cout<<#x<<": "<<x<<endl
#define fcout(n) cout<<fixed<<setprecision((n))
#define scout(n) cout<<setw(n)
#define vary(type,name,size,init) vector< type> name(size,init)

#define rep(i,n) for(int i = 0; i < (int)(n);++i)
#define REP(i,a,b) for(int i = (a);i < (int)(b);++i)
#define repi(it,array) for(auto it = array.begin(),end = array.end(); it != end;++it)
#define repa(n,array) for(auto &n :(array))

using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using dict = map<string,int>;
using pii = pair<int,int> ;

constexpr int imax = ((1<<30)-1)*2+1 ;
constexpr int inf = 100000000;
constexpr double PI = acos(-1.0) ;
double eps = 1e-10 ;
const int dy[] = {-1,0,1,0};
const int dx[] = {0,-1,0,1};

inline bool value(int x,int y,int w,int h){
  return (x >= 0 && x < w && y >= 0 && y < h);
}

template<typename T>
void Unique(vector<T> &v){
  sort(all(v));
  v.erase(unique(all(v)),v.end());
}

template<typename T>
T ston(string& str, T n){
  istringstream sin(str) ;
  T num ;
  sin >> num ;
  return num ;
}

void Ans(bool f){
  if(f) cout << "YES"<<endl;
  else cout << "NO"<<endl;
}

int is_leap(int y){
  if(y % 400 == 0) return 366;
  else if(y % 100 == 0) return 365;
  else if(y % 4 == 0) return 366;
  return 365;
}
const int days[12] = {31,28,31,30,31,30,31,31,30,31,30,31};
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  int y,y2,d,d2,m,m2;
  while(cin >> y >> m >> d >> y2 >> m2 >> d2){
    if(y < 0 || m < 0 || d < 0 || y2 < 0|| m2 < 0|| d2 < 0) break;
    ll ans = 0;
    rep(i,m){
      ans -= days[i];
    }
    if(is_leap(y) == 366 && m > 2) --ans;
    ans -= d;
    while(y2 - y != 0){
      ans += is_leap(y);
      ++y;
    }
    rep(i,m2){
      ans += days[i];
    }
    if(is_leap(y2) == 366 && m2 > 2) ++ans;
    ans += d2;
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

static int month[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

int uruu(int y, int day)
{
	if (y % 4 == 0 && y % 100 != 0 || y % 400 == 0){
		day++;
	}
	return (day);
}

int main()
{
	int y1, y2, m1, m2, d1, d2;
	
	while (cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2){
		int day = 0;
		for (int i = y1; i < y2; i++){
			day = uruu(i, day);
		}
		day += (y2 - y1) * 365;
		for (int i = m1 + 1; i < m2; i++){
			day += month[i];
		}
		if (m1 == m2){
			day += (d2 -d1);
		}
		else {
			day += month[m1] - d1 + d2;
		}
		cout << day << endl;
	}
	return (0);
}

C++: #include <iostream>
using namespace std;int main(){int a,b,c,d,e,f,g;int h[13]={0,0,31,59,90,120,151,181,212,243,273,304,334};while(1){cin>>a>>b>>c>>d>>e>>f;if(a==-1)break;g=f-c+h[e]-h[b]+(d-a)*365;if(e<3)d--;if(b<3)a--;g+=d/4-a/4-d/100+a/100+d/400-a/400;cout<<g<<endl;}return 0;}