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>
using namespace std;

bool isLeap(int y){
	return y % 4 == 0 && (y % 100 != 0 || y % 400 == 0);
}

long long toDay(int y,int m,int d){
	long long res = d;

	res += (y-1) * 365;
	for(int i=4;i<y;i++){
		if(isLeap(i)) res++;
	}

	for(int i=1;i<m;i++){
		switch(i){
		case 2:
			if(isLeap(y)) res += 29;
			else res += 28;
			break;
		case 4:
		case 6:
		case 9:
		case 12:
			res += 30;
			break;
		default:
			res += 31;
			break;
		}
	}

	return res;
}

int main(void){
	int y1,m1,d1,y2,m2,d2;

	while(cin>>y1>>m1>>d1>>y2>>m2>>d2 && y1!=-1){
		cout<<toDay(y2,m2,d2) - toDay(y1,m1,d1)<<endl;
	}

	return 0;
}

C++: #include <vector>
#include <list>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <queue>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <cstdio>
#include <climits>
#include <complex>
#include <cstdint>
#include <tuple>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int N, K;
struct Edge {
    int to, cost;
    Edge(int to, int cost): to(to), cost(cost) {}
};

typedef vector<vector<Edge>> AdjList;
AdjList graph;

const int INF = 100000000;

vi dist;
int d[110][110];

bool bellman_ford(int n, int s) { // nは頂点数、sは開始頂点
    dist = vector<int>(n, INF);
    dist[s] = 0; // 開始点の距離は0
    for (int i = 0; i < n; i++) {
        for (int v = 0; v < n; v++) {
            for (int u = 0; u < n; u++) {
                if (dist[v] != INF && dist[u] > dist[v] + d[v][u]) {
                    dist[u] = dist[v] + d[v][u];
                    if (i == n - 1) return true; // n回目にも更新があるなら負の閉路が存在
                }
            }
        }
    }
    return false;
}
bool cmp(const pair<int,string>& a, const pair<int,string>& b){
    return a.first < b.first;
}

const int uruu = 366;

int days[13]={0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int main() {
    //cout << fixed << setprecision(15);

    
    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;
        
        LL ub=0, lb=0;
        
        for(int i = 1; i<=y1-1; ++i){
            if(i%4 == 0 && (i%100 != 0 || i%400 == 0)) lb += 366;
            else lb += 365;
        }
        
        if(y1%4 == 0 && (y1%100 != 0 || y1%400 == 0)) days[2]++;
        for(int i=1; i<=m1; ++i){
            for(int j=1; j<= ((i==m1)?d1:days[i]); ++j) lb++;
        }
        
        days[2] = 28;
        
        for(int i = 1; i<=y2-1; ++i){
            if(i%4 == 0 && (i%100 != 0 || i%400 == 0)) ub += 366;
            else ub += 365;
        }
        
        if(y2%4 == 0 && (y2%100 != 0 || y2%400 == 0)) days[2]++;
        for(int i=1; i<=m2; ++i){
            for(int j=1; j<= ((i==m2)?d2:days[i]); ++j) ub++;
        }
        
        cout << ub-lb << endl;
        
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

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;
		}
	} else {
		return false;
	}
}

int w_month(int m, int y) {
	int montha[] = {4, 6, 9, 11};
	if (m == 2) {
		if (is_leap(y)) {
			return 3;
		} else {
			return 2;
		}
	} else {
		for (int i = 0; i < 4; i++) {
			if (m == montha[i]) {
				return 1;
			}
		}
		return 0;
	}
}

int main() {
	int y1, m1, d1, y2, m2, d2;
	const int year = 31 * 7 + 30 * 4 + 28;
	const int month[] = {31, 30, 28, 29};
	while (cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2) {
		if (y1 < 0 || m1 < 0 || d1 < 0 || y2 < 0 || m2 < 0 || d2 < 0 ) {
			break;
		}
		int count = 0;
		if (y1 == y2) {
			if (m1 != m2) {
				for (int i = m1+1; i < m2; i++) {
					count += month[w_month(i, y1)];
				}
				count += d2;
				count += month[w_month(m1, y1)]-d1;
			} else {
				count = d2-d1;
			}
		} else {
			if (y2 - y1 > 1) {
				count += year * (y2-y1-1);
				for (int i = y1+1; i <= y2-1; i++) {
					if (is_leap(i)) {
						count++;
					}
				}
			}
			for (int i = m1+1; i < 13; i++) {
				count += month[w_month(i, y1)];
			}
			for (int i = 1; i < m2; i++) {
				count += month[w_month(i, y2)];
			}
			count += d2;
			count += month[w_month(m1, y1)]-d1;
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<list>
#include<string>
#include<map>
#include<vector>
using namespace std;
bool step(int y2) {
 if ((y2) % 4 == 0) {
  if ((y2) % 100 == 0) {
   if ((y2) % 400 == 0)
    return true;
   else
    return false;
  }
  else
   return true;
 }
 else
  return false;
}
int main(){
 int y1, m1, d1, y2, m2, d2;
 int m[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };
 while (cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2 && y1>=0) {
  int y3 = y2;
  if(y2<y1){
	  swap(y1,y2);
	  swap(m1,m2);
	  swap(d1,d2);
  }
  else if(y2==y1){
	  if(m2<m1){
		  swap(m1,m2);
		  swap(d1,d2);
	  }
	  else if(m2==m1){
		  if(d2<d1){
			  swap(d1,d2);
		  }
	  }
  }
  while (y2 != y1) {
   y2--;
   if (step(y2)&&y2!=y1)
    d2 += 366;
   else
    d2 += 365; 
  }
  if (step(y3) && m2 >= 2)
   d2++;
  while (m2 < m1) {
   m2++;
   if (m2 == 2 && step(y1))
    d2--;
   d2 -= m[m2-1];
  }
  while (m2 > m1) {
   m2--;
   if (m2 == 2 && step(y1))
    d2++;
   d2 += m[m2 - 1];
  }
  d2 -= d1;
  cout <<d2<< endl;
 }
}