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 <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>

#define month(m, a) \
		for(int i = 1; i <= m; i++)\
		if(i == 2){\
			a += 28;\
		} else if(i == 4 || i == 6 || i == 9 || i == 11){\
			a += 30;\
		} else {\
			a += 31;\
		}\

using namespace std;

int main(int argc, char **argv)
{
	int y1, m1, d1, y2, m2, d2, a1, a2, a3, a4;
	while(1){
		cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2;
		if(y1 == -1){
			break;
		}
		if(y1==y2&&m1==m2&&d1==d2){
			cout << "0" << endl;
			continue;
		}
		
		a2 = y2 - y1;
		for(a1 = 0; y1 <= y2; y1++){
			if(y1%400 == 0 || (y1%4 == 0 && y1%100 != 0)){
				a1 += 1;
				y1 += 4;
				break;
			}
		}
		for(; y1 <= y2; y1 += 4){
			if(y1%400 == 0 || y1 % 100 != 0){
				a1 += 1;
			}
		}
		a2 -= a1;
		a4 = a3 = 0;
		month(m1, a3);
		month(m2, a4);

		cout << (a2*365 + a1*366 + a4 - a3 + d2 - d1) << endl;
	}
	
	return 0;
}

C++: #include <iostream>

using namespace std;


int month[12]={31,28,31,30,31,30,31,31,30,31,30,31};
int l_month[12]={31,29,31,30,31,30,31,31,30,31,30,31};

bool leap(int year) {

  if((year%4==0 && year%100!=0) || year%400==0) return true;

  else return false;

}

int getdaycount(int year,int month,int day) {

  int days;

  if(month<=2) {

    year--;

    month+=12;

  }

  days=(365*year+year/4-year/100+year/400+306*(month+1)/10+day-428);

  return days;

}

int main() {

  int y1,m1,d1,y2,m2,d2;

  int day1,day2;


  while(cin >> y1 >> m1 >> d1 >> y2 >> m2 >> d2) {

    if(y1==-1 || m1==-1 || d1==-1 || y2==-1 || m2==-1 || d2==-1) break;

    day1=getdaycount(y1,m1,d1);

    day2=getdaycount(y2,m2,d2);

    cout << day2-day1 << endl;

  }

  return 0;

}

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <string>
#include <queue>
#include <vector>
#include <sstream>
#include <functional>
using namespace std;
int month[12]={30,28,31,30,31,30,31,31,30,31,30,31};
int count(int y,int m,int d){//1/1/1~y/m/d?????\??°(1/1/1=1)
	int num=0;
	if((!(y%4)&&(y%100))||(!(y%400)))month[1]=29;
	for(int i=0;i<m-1;i++){
		num+=month[i];
	}
	num+=d;
	month[1]=28;
	return num;
}
int main()
{
	int y1,m1,d1,y2,m2,d2;
	while(cin>>y1>>m1>>d1>>y2>>m2>>d2){
		if(y1<0||m1<0||d1<0||y2<0||m2<0||d2<0)break;
		int ans=0;
		if(y1==y2){
			ans=count(y2,m2,d2)-count(y1,m1,d1);
		}else if(y2-y1==1){
			ans=365-count(y1,m1,d1)+count(y2,m2,d2);
			if((!(y1%4)&&(y1%100))||(!(y1%400)))ans++;
		}else{
			y1++;y2--;
			ans+=(y2-y1+1)*365+(y2/4-y2/100+y2/400)-((y1-1)/4-(y1-1)/100+(y1-1)/400);
			ans+=count(++y2,m2,d2)+365-count(--y1,m1,d1);
			if((!(y1%4)&&(y1%100))||(!(y1%400)))ans++;
		}
		cout<<ans<<endl;

	}
    return 0;
}