Go: package main

import (
	"bufio"
		"os"
	"strings"
	"strconv"
		"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)))
		fmt.Println(ut2jd(ele_t2[0], ele_t2[1], ele_t2[2])-ut2jd(ele_t1[0], ele_t1[1], ele_t1[2]))
	}
}

func ut2jd(year, month, day int) int {
	if month <= 2 {
		year--
		month += 12
	}
	s := 3 + year/4 - year/100 + year/400
	s += 1720994 + year*365 + (month+1)*30 + (month+1)*3/5 + day
	return s
}

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;
}




long long ly[55];
long long ry[55];
long long lm[55];
long long rm[55];
long long ld[55];
long long rd[55];

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){
        long long 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)
        ans += (mimos[rm[i]-1] + (rd[i] - 1));
        if (ry[i] % 4 == 0){
            if (rm[i] > 2){
                ans++;
                if (ry[i] % 100 == 0){
                    ans--;
                    if (ry[i] % 400 == 0){
                        ans++;
                    }
                }
            }
        }
        
        for (long long j = ly[i]; j < ry[i]; j++){
            if (j % 4 == 0){
                ans += ((ry[i] - j - 1) / 4) + 1;
                break;
            }
        }

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

        for (long long 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>
using namespace std;
int main(){
  int y1,m1,d1,y2,m2,d2,day,i=0;
  int month[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
  for(;i<12;month[i+1]+=month[i],i++);
  while(true){
    cin>>y1>>m1>>d1>>y2>>m2>>d2;
    if (y1==-1)break;
    day=d2-d1+month[m2-1]-month[m1-1]+(y2-y1)*365;
    if(m2<3)y2--;
    if(m1>2)y1++;
    day+=y2/4-(y1-1)/4-y2/100+(y1-1)/100+y2/400-(y1-1)/400;
    cout<<day<<endl;
  }
  return 0;
}

C++: #include <iostream>
using namespace std;
#define rep2(x,from,to) for(int x=(from);x<(to);++(x))
#define rep(x,to) rep2(x,0,to)
int d[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int calc(int y1, int y2) {
	int a;
	a = y2/4-y1/4;
	if(y1%4==0) a++;
	a -= y2/100-y1/100;
	if(y1%100==0) a--;
	a += y2/400-y1/400;
	if(y1%400==0) a++;
	return a;
}
bool uruu(int y) {
	return y%4 == 0 && y%100 != 0 || y%400 == 0;  
}
int main() {
	int y1, m1, d1, y2, m2, d2;
	while(cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2 && (y1>=0 && m1>=0 && d1>=0 && y2>=0 && m2>=0 && d2>=0)) {
		int uy = calc(y1,y2);
		int ret = (y2-y1+1)*365+uy;
		int day = 0;
		rep(i,m1-1) day += d[i];
		for(int i=11; i>=m2; --i) day += d[i];
		day += d1-1, day += d[m2-1]-d2+1;
		if(uruu(y1) && m1>2) day++;
		if(uruu(y2) && m2<=2) day++;
		cout << ret - day << endl;
	}
	return 0;
}

C++: //#include <bits/stdc++.h>
#include <iostream>
#define rep(i,l,n) for(int i=l;i<n;i++)
#define rer(i,l,n) for(int i=l;i<=n;i++)
#define all(a) a.begin(),a.end()
#define o(a) cout<<a<<endl
#define fi first
#define se second
using namespace std;
typedef long long ll;
//typedef vector<int> vint;
typedef pair<int,int> pii;

int y1,m1,d1,y2,m2,d2;
int md[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
int mdl[13]={0,31,29,31,30,31,30,31,31,30,31,30,31};

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

int days(int y,int m,int d){
	int ans=0,cm=1;
	while(1){
		if(cm==m){
			ans+=d; break;
		}
		else if(leap_year(y)) ans+=mdl[cm];
		else ans+=md[cm];
		cm++;
	}
	return ans;
}

int main(){
	while(1){
		int ans=0;
		cin>>y1>>m1>>d1>>y2>>m2>>d2;
		if(y1<0||m1<0||d1<0||y2<0||m2<0||d2<0) break;
		while(1){
			if(y1==y2) break;
			else if(leap_year(y1)) ans+=366;
			else ans+=365;
			y1++;
		}
		ans+=days(y2,m2,d2)-days(y1,m1,d1);
		o(ans);
	}
}