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 <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 (ry[i] % 4 == 0){
            if (rm[i] > 4){
                ans++;
                if (ry[i] % 100 == 0){
                    ans--;
                    if (ry[i] % 400 == 0){
                        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<string>
#include<vector>
using namespace std;

int main()
{
	while (1)
	{
		int L, R; L = R = 0;
		int a, b, c, d, e, f;
		cin >> a >> b >> c >> d >> e >> f;
		if (a == -1 && b == -1 && c == -1 && d == -1 && e == -1 && f == -1)break;
		for (int g = 1; g < a; g++)
		{
			if (g % 400 == 0) L += 366;
			else if (g % 100 == 0) L += 365;
			else if (g % 4 == 0)L += 366;
			else L += 365;
		}
		
		for (int g = 1; g < d; g++)
		{
			if (g % 400 == 0) R += 366;
			else if (g % 100 == 0) R += 365;
			else if (g % 4 == 0)R += 366;
			else R += 365;
		}
		
		for (int h = 1; h < b; h++) {
			switch (h) {
			case 1:case 3:case 5:case 7:case 8:case 10:case 12:L += 31; break;
			case 4:case 6:case 9:case 11:L += 30; break;
			case 2:if (a % 400 == 0) L += 29;
				   else if (a % 100 == 0) L += 28;
				   else if (a % 4 == 0)L += 29;
				   else L += 28;
			}
			}
			
			for (int h = 1; h < e; h++) {
				switch (h) {
				case 1:case 3:case 5:case 7:case 8:case 10:case 12:R += 31; break;
				case 4:case 6:case 9:case 11:R += 30; break;
				case 2:if (a % 400 == 0)R += 29;
					   else if (a % 100 == 0) R += 28;
					   else if (a % 4 == 0)R += 29;
					   else R += 28;
				}
			}
			
		cout << (R + f) - (L + c) << endl;
	}
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <stack>
#include <queue>
using namespace std;
 
//          1  2  3  4  5  6  7  8  9 10 11 12
int h[12] = {31,28,31,30,31,30,31,31,30,31,30,31};
 
bool is_leap(int y){
	if( y % 4 == 0 ){
		if( y % 100 == 0 ){
			if( y % 400 == 0 ){
				return true;
			}else{
				return false;
			}
		}else{
			return true;
		}
	}
	return false;
}
 
struct Time{
	int y,m,d;
	Time(int y_, int m_, int d_){
		y = y_;
		m = m_;
		d = d_;
	}
	void inc(){
		if( is_leap( y ) && m == 2 && d == 28 ){
			d++;
		}else if(is_leap( y ) && m == 2 && d == 29 ){
			m++;
			d = 1;
		}else{
			if( d < h[m-1] ){
				d++;
			}else{
				d = 1;
				if( m == 12 ){
					m = 1;
					y++;
				}else{
					m++;
				}
			}
		}
	}
	bool equal(Time t){
		return ( y == t.y && m == t.m && d == t.d );
	}
};
 
int main(){
	int y1, m1, d1, y2, m2, d2;
	while( cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2 ){
		if( y1 == -1 || m1 == -1 || d1 == -1 || y2 == -1 || m2 == -1 || d2 == -1 ) break;
		Time t1( y1, m1, d1 );
		Time t2( y2, m2, d2 );
		int cnt = 0;
		while( !t1.equal(t2) ){
			t1.inc();
			cnt++;
		}
		cout << cnt << endl;
	}
}