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 <stdio.h>

int is_leap_year( int year )
{
	if ( year % 4 == 0 )
	{
		if ( year % 100 != 0 ) return 1;
		else if ( year % 400 == 0 ) return 1;
	}
	return 0;
}

int main( void )
{
	int days_of_month[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
	int yy_1, mm_1, dd_1;
	int yy_2, mm_2, dd_2;
	int leap_years;
	int days;
	int i;

	while ( 1 )
	{
		scanf( "%d%d%d", &yy_1, &mm_1, &dd_1 );
		scanf( "%d%d%d", &yy_2, &mm_2, &dd_2 );

		if ( yy_1 == -1 || mm_1 == -1 || dd_1 == -1 || yy_2 == -1 || mm_2 == -1 || dd_2 == -1 ) break;

		leap_years = 0;
		days = 0;

		for ( i = yy_1 + 1; i < yy_2; i++ )
		{
			if ( is_leap_year( i ) ) leap_years++;
		}

		if ( yy_2 - yy_1 > 1 )
		{
			days += ( yy_2 - yy_1 - 1 ) * 365 + leap_years;
		}

		/* yy_1, yy_2ÉÂ¢Ä */
		if ( yy_1 == yy_2 )
		{
			if ( mm_1 == mm_2 )
			{
				days += dd_2 - dd_1;
			}
			else
			{
				if ( is_leap_year( yy_1 ) )
				{
					if ( ( mm_1 == 1 || mm_1 == 2 ) && mm_2 > 2 ) days++;
				}
				for ( i = mm_1 + 1; i <= mm_2 - 1; i++ ) days += days_of_month[i];
				days += days_of_month[mm_1] - dd_1;
				days += dd_2;
			}
		}
		else
		{
			if ( is_leap_year( yy_1 ) )
			{
				if ( mm_1 == 1 || mm_1 == 2 ) days++;
			}
			if ( is_leap_year( yy_2 ) )
			{
				if ( mm_2 > 2 ) days++;
			}
			for ( i = mm_1 + 1; i <= 12; i++ ) days += days_of_month[i];
			for ( i = 1; i <= mm_2 - 1; i++ ) days += days_of_month[i];
			days += days_of_month[mm_1] - dd_1;
			days += dd_2;
		}

		printf( "%d\n", days );
	}

	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>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

int main()
{
    int month[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    for(;;){
        int y[2], m[2], d[2];
        for(int i=0; i<2; ++i){
            cin >> y[i] >> m[i] >> d[i];
            if(y[i] < 0 || m[i] < 0 || d[i] < 0)
                return 0;
        }

        long long sum[2] = {0};
        for(int i=0; i<2; ++i){
            for(int j=1; j<y[i]; ++j){
                sum[i] += 365;
                if(j % 400 == 0 || (j % 100 != 0 && j % 4 == 0))
                    ++ sum[i];
            }
            for(int j=1; j<m[i]; ++j){
                sum[i] += month[j];
                if(j == 2 && (y[i] % 400 == 0 || (y[i] % 100 != 0 && y[i] % 4 == 0)))
                    ++ sum[i];
            }
            sum[i] += d[i];
        }

        cout << (sum[1] - sum[0]) << endl;
    }
}

C++: #include<iostream>
using namespace std;
int Q(int y,int m,int d) {
  if (m==1||m==2){--y;m+=12;}
   return 365*y+y/4-y/100+y/400+(306*(m+1))/10+d-428;
}
int main(){
  int a,b,c,d,e,f,g;
  while(cin>>a>>b>>c>>d>>e>>f){
    cout<<Q(d,e,f)-Q(a,b,c)<<endl;
  }
}