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++: #define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;

#define int long long
#define DBG 1
#define dump(o) if(DBG){cerr<<#o<<" "<<o<<endl;}
#define dumpc(o) if(DBG){cerr<<#o; for(auto &e:(o))cerr<<" "<<e;cerr<<endl;}
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define each(it,c) for(auto it=(c).begin();it!=(c).end();it++)
#define all(c) c.begin(),c.end()
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9 + 7);
vector<int>year(1, 0), month{ 0,0,31,28,31,30,31,30,31,31,30,31,30,31 }, day;
int YEAR(int n) {
	if (n < year.size())return year[n];

	int x = year.size() - 1;
	while (year.size() <= n) {
		if (x % 4 == 0 && (x % 100 != 0 || x % 400 == 0))year.push_back(year[x] + 366);
		else year.push_back(year[x] + 365);
		x++;
	}

	return year[n];
}
int Month(int y, int m) {
	int res = month[m];
	if (y % 4 == 0 && (y % 100 != 0 || y % 400 == 0)) {
		if(m>=3)res++;
	}
	return res;
}
signed main() {
	rep(i, 1, month.size())month[i] += month[i - 1];
	while (1) {
		int y1, y2, m1, m2, d1, d2;
		cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2;
		if (y1 < 0 || y2 < 0 || m1 < 0 || m2 < 0 || d1 < 0 || d2 < 0)break;

		cout << YEAR(y2) - YEAR(y1) + Month(y2, m2) - Month(y1, m1) + d2 - d1 << endl;;
	}
	return 0;
}

C++: #include <iostream>
#include <fstream>
#include <string> 
#include <cstring>
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <vector>
#include <algorithm>
#include <numeric>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <functional>
#include <cctype>
#include <list>
//#include <boost/multiprecision/cpp_int.hpp>

#define BIT(a) (1 << (a))
#define EPS (1e-10)

using namespace std;
//using namespace boost::multiprecision;

const long long MOD = 1000000007;
const int COUNTER_CLOCKWISE = 1;
const int CLOCKWISE = -1;
const int ONLINE_BACK = 2;
const int ONLINE_FRONT = -2;
const int ON_SEGMENT = 0;

class Point{
    public:
    double x, y;

    Point(double x = 0, double y = 0): x(x), y(y) {}
    
    Point operator+(Point p){return Point(x + p.x, y + p.y);}
    Point operator-(Point p){return Point(x - p.x, y - p.y);}
    Point operator*(double a){return Point(a * x, a * y);}
    Point operator/(double a){return Point(x / a, y / a);}

    bool operator < (const Point &p) const {
        return y != p.y ? y < p.y : x < p.x;
    }

    double norm(){return x * x + y * y;}
};
typedef Point Vector;
typedef vector<Vector> Polygon;

double cross(Vector a, Vector b){
    return a.x*b.y - a.y*b.x;
}

double dot(Vector a, Vector b){
    return a.x * b.x + a.y * b.y;
}

int ccw(Point p0, Point p1, Point p2){
    Vector a = p1 - p0;
    Vector b = p2 - p0;
    if (cross(a, b) > EPS)return COUNTER_CLOCKWISE;
    if (cross(a, b) > -EPS)return CLOCKWISE;
    if (dot(a, b) < -EPS)return ONLINE_BACK;
    if (a.norm() < b.norm())return ONLINE_FRONT;

    return ON_SEGMENT;
}

Polygon andrewScan(Polygon s){
    Polygon u, l;
    if (s.size() < 3) return s;
    sort(s.begin(), s.end());
    u.push_back(s[0]);
    u.push_back(s[1]);
    l.push_back(s[s.size()-1]);
    l.push_back(s[s.size()-2]);
    
    for (size_t i = 2; i < s.size(); i++){
        for (size_t n = u.size(); n >= 2 && ccw(u[n-2], u[n-1], s[i]) == COUNTER_CLOCKWISE; n--){
            u.pop_back();
        }
        u.push_back(s[i]);
    }

    for (int i = (int)s.size() - 3; i >= 0; i--){
        for (size_t n = l.size(); n >= 2 && ccw(l[n-2], l[n-1], s[i]) == COUNTER_CLOCKWISE; n--){
            l.pop_back();
        }
        l.push_back(s[i]);
    }

    reverse(l.begin(), l.end());
    for (size_t i = u.size() - 2; i >= 1; i--) l.push_back(u[i]);

    return l;
}

long long mod_pow(long long x, long long n){
    long long res = 1;
    for(int i = 0;i < 60; i++){
        if(n >> i & 1) res = res * x % MOD;
        x = x * x % MOD;
    }
    return res;
}


int ly[51];
int ry[51];
int lm[51];
int rm[51];
int ld[51];
int rd[51];

int mimos[13];

int  init(){
    int month[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    for (int i = 1; i < 13; i++){
        mimos[i] = month[i] + mimos[i-1];
    }
    return 0;
}


int main(void){
    int cnt = 0;
    init();
    while (1){
        int tmp[6];
        int flag = 0;
        for (int i = 0; i < 6; i++){
            cin >> tmp[i];
            if (tmp[i] == -1) flag = 1;
        }
        if (flag) break;
        ly[cnt] = tmp[0];
        lm[cnt] = tmp[1];
        ld[cnt] = tmp[2];
        ry[cnt] = tmp[3];
        rm[cnt] = tmp[4];
        rd[cnt] = tmp[5];
        cnt++;
    }

    for (int i = 0; i < cnt; i++){
        long long ans = 0;
        ans += (ry[i] - ly[i])*365;
        ans += (-mimos[lm[i]-1] - (ld[i] - 1) + mimos[rm[i]-1] + (rd[i]-1));
        if (!ly[i] % 4){
            if (lm[i] > 4 ){
                ans++;
                if (!ly[i] % 100){
                    ans--;
                    if (!ly[i] % 400){
                        ans++;
                    }
                }
            }
        }
        
        for (int j = ly[i]; j < ry[i]; j++){
            if (j % 4 == 0){
                ans += ((ry[i] - j - 1) / 4) + 1;
                break;
            }
        }

        for (int j = ly[i]; j < ry[i]; j++){
            if (j % 100 == 0){
                ans -= (((ry[i] - 1 - j) / 100) + 1);
                break;
            }
        }

        for (int j = ly[i]; j < ry[i]; j++){
            if (j % 400 == 0){
                ans += (((ry[i] - 1 - j) / 400) + 1);
                break;
            }
        }

        cout << ans << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cstring>
#include<ctime>
using namespace std;
int main(void){
	while( true ){
		int y1, m1, d1;
		int y2, m2, d2;
		struct tm t1, t2;
		double diff;
		time_t second1, second2;

		cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2;
		if( y1 == -1 || y2 == -1 || m1 == -1 || m2 == -1 || d1 == -1 || d2 == -1 )
			break;

		memset(&t1,0,sizeof(t1));
		memset(&t2,0,sizeof(t2));
		t1.tm_year = y1 - 1900;
		t1.tm_mon = m1 - 1;
		t1.tm_mday = d1;
		t2.tm_year = y2 - 1900;
		t2.tm_mon = m2 - 1;
		t2.tm_mday = d2;

		second1 = mktime( &t1 );
		second2 = mktime( &t2 );

		diff = difftime( second2, second1 );
		
		cout << (int)(diff / (3600*24)) << '\n';
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int getday(int, int, int);

int main(void)
{
  int y1, m1, d1, y2, m2, d2;
  int day1, day2;
  int year;
  
  while(1) {
    cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2;
    if(y1 < 0 || m1 < 0 || d1 < 0 || y2 < 0 || m2 < 0 || d2 < 0) break;
    day1 = getday(y1, m1, d1);
    day2 = getday(y2, m2, d2);
    
    cout << day2 - day1 << endl;
  }
  return 0;
}

int getday(int y, int m, int d)
{
  int day = 0,u;

  u = (y % 4 == 0 && y % 100 != 0 || y % 400 == 0);

  for(int i = 1 ; i < y ; i++) {
    day += 365;
    if(i % 4 == 0 && i % 100 != 0 || i % 400 == 0) day += 1;
      }

  for(int i = 1 ; i < m ; i++) {
    day += 30;
    if(i == 1 || i == 3 || i == 5 || i == 7 || i == 8 || i == 10 || i == 12) day++;
    if(i == 2) { day -= 2; if(u) day++; }
  }
  for(int i = 0 ; i < d ; i++) {
    day++;
  }
  return day;
}