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;
}




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 && rm[i] > 2){
            ans++;
            if (ry[i] % 100 == 0 && ry[i] % 400){
                ans--;
            }
        }
        
        for (long long j = ly[i]; j < ry[i]; j++){
            if (j == ly[i] && (lm[i] > 2 || (lm[i] == 2 && ld[i] == 29))) 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 == ly[i] && (lm[i] > 2 || (lm[i] == 2 && ld[i] == 29))) j++;
            if (lm[i] < 2) 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 == ly[i] && (lm[i] > 2 || (lm[i] == 2 && ld[i] == 29))) j++;
            if (lm[i] < 2) j++;
            if (j % 400 == 0){
                ans += (((ry[i] - 1 - j) / 400) + 1);
                break;
            }
        }

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

C++: #include <cstdio>
using namespace std;

int main()
{
	int y1 = 0, m1, d1, y2, m2, d2;
	
	//2 4 6 9 11
	
	while(1){
		int ans = 0;
		int man[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
		scanf("%d %d %d %d %d %d", &y1, &m1, &d1, &y2, &m2, &d2);
		if (y1 < 0){
			break;
		}
		if (y1 == y2){
			if (y1 % 4 == 0 && y1 % 100 != 0 || y1 % 400 == 0){
				man[1]++;
			}
			
			if (m1 == m2){
				ans += d2 - d1;
			}
			else {
				while (m1 < m2){
					ans += man[m1 - 1];
					m1++;
				}
				if (d1 > d2){
					ans -= man[m2] - (d1 - d2);
				}
				else {
					ans += d2 - d1;
				}
			}
		}
		else {
			while (y1 < y2){
				if (y1 % 4 == 0 && y1 % 100 != 0 || y1 % 400 == 0){
					ans += 366;
				}
				else {
					ans += 365;
				}
				y1++;
			}
			if (y2 % 4 == 0 && y2 % 100 != 0 || y2 % 400 == 0){
				man[1]++;
			}
			while (m1 < m2){
				ans += man[m1 - 1];
				m1++;
			}
			while (m2 < m1){
				ans -= man[m2 - 1];
				m2++;
			}
			if (d1 > d2){
				ans -= man[m2 - 1] - (d1 - d2);
			}
			else {
				ans += d2 - d1;
			}
		}
		printf("%d\n", ans);
	}
	
	return (0);
}

C++: #include<iostream>
using namespace std;

int main()
{
  int y1,m1,d1,y2,m2,d2;
  int m[13]={0,31,28,31,30,31,30,31,31,30,31,30,31},i;
  int days1,days2,difference;
  for(i=1;i<12;i++)m[i]+=m[i-1];
  while(cin>>y1>>m1>>d1>>y2>>m2>>d2,~y1&~m1&~d1&~y2&~m2&~d2){
    y1--;y2--;
    days1=365*y1+m[m1-1]+d1+y1/4-y1/100+y1/400;
    days2=365*y2+m[m2-1]+d2+y2/4-y2/100+y2/400;
    y1++;y2++;
    if((y1%4==0 && y1%100!=0)||(y1%400==0))if((m1==2&&d1==29)||(m1>=3))days1++;
    if((y2%4==0 && y2%100!=0)||(y2%400==0))if((m2==2&&d2==29)||(m2>=3))days2++;
    cout<<days2-days1<<endl;
  }
}