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<cstdio>
#include<iostream>
#include<map>
using namespace std;
inline bool check(int year) {
	return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}
int cnt(int year) {
	int ans = 0;
	for(int i = 1; i <= year; i++) {
		if(check(i)) ans++;
	}
	return ans;
}
int main() {
	int d[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	for(int i = 2; i <= 12; i++) {
		d[i] += d[i-1];
	}
	int y1,m1,d1,y2,m2,d2;
	while(scanf("%d%d%d%d%d%d",&y1,&m1,&d1,&y2,&m2,&d2)) {
		if(y1 == -1 || m1 == -1 || d1 == -1 || y2 == -1 || m2 == -1 || d2 == -1) break;
		int a1 = 365 * y1 + cnt(y1 - 1) + d[m1-1] + d1;
		int a2 = 365 * y2 + cnt(y2 - 1) + d[m2-1] + d2;
		printf("%d\n",a2 - a1);
	}
	
	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[] = {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[rm[i]] - mimos[lm[i]] - ld[i] + rd[i]);
        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>
using namespace std;
int h[] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
int l[] = { 0,31,29,31,30,31,30,31,31,30,31,30,31 };
bool isLeap(int y)
{
  if(y % 400 == 0) return true;
  else if(y % 100 == 0) return false;
  else if(y % 4 == 0) return true;
  else return false;
}
int daysfrom(int y,int m,int d)
{
  int *p = isLeap(y) ? l : h;
  int s = p[m] - d + 1;
  int i;
  for(i=m+1; i<13; i++){
    s += p[i];
  }
  return s;
}
int daysto(int y,int m,int d)
{
  int *p = isLeap(y) ? l : h;
  int s = d - 1;
  int i;
  for(i=m-1; i>0; i--){
    s += p[i];
  }
  return s;
}
int main()
{
  int y1,m1,d1,y2,m2,d2;
  for(;;){
    cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2;
    if(y1<0 || m1<0 || d1<0 || y2<0 || m2<0 || d2<0) break;
    if(y1==y2){
      int s1 = daysto(y1,m1,d1);
      int s2 = daysto(y2,m2,d2);
      cout << s2 - s1 << endl;
    }else{
      int s = daysfrom(y1,m1,d1);
      int i;
      for(i=y1+1; i<y2; i++){
	s += daysfrom(i,1,1);
      }
      s += daysto(y2,m2,d2);
      cout << s << endl;
    }
  }
  return 0;
}

C++: #include <stdio.h>

int main(void)
{
	int y, m, d, y2, m2, d2;
	int i, j;
	int ans;
	char month[12] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	
	while (1){
		scanf("%d %d %d %d %d %d", &y, &m, &d, &y2, &m2, &d2);
		
		if (y < 0 || m < 0 || d < 0 || y2 < 0 || m2 < 0 || d2 < 0){
			break;
		}
		
		ans = 0;
		
		while (1){
			
			if (y == y2 && d == d2 && m == m2){
				break;
			}
			
			ans++;
			d++;
			
			if (d > month[m - 1]){
				d = 1;
				m++;
			}
			if (m > 12){
				m = 1;
				y++;
			}
			
			if ( (y % 4 == 0 && y % 100 != 0) || y % 400 == 0){
				month[1] = 29;
			}
			else {
				month[1] = 28;
			}
		}
		
		printf("%d\n", ans);
	}
	
	return (0);
}