C++ :

#include <cstdio>
int main(void) {
	int y0, m0, d0, y, m, d, s=0, i;
	int md[12]={31,28,31,30,31,30,31,31,30,31,30,31};
	scanf("%d%d%d%d%d%d", &y0, &m0, &d0, &y, &m, &d);
	y0 += 19; //目标年份，第20个生日，只需要过19年
	if(y%400==0 || y%4==0&&y%100>0) md[1] = 29;
	for(s=-d, i=0; i<m-1; ++i) s -= md[i]; // 负数表示当年已过的天数
	for(; y<y0; ++y) { //中间年份的天数
		if(y%400==0 || y%4==0&&y%100>0) s += 366;
		else s += 365;
	}
	if(y%400==0 || y%4==0&&y%100>0) md[1] = 29;
	else  md[1] = 28;
	for(i=0, s+=d0; i<m0-1; ++i) s += md[i]; // 加上目标年份的天数
	printf("%d\n", s);
	return 0;
}

Pascal :

var y,m,d:longint;
    y0,m0,d0:longint;
    s,i,p,q:longint;

function year(n:longint):longint;
begin
  if (n mod 4=0) and (n mod 100<>0)or(n mod 400=0) then
    year:=366
  else
    year:=365;
end;

function month(n,nian:longint):longint;
begin
  case n of
    1,3,5,7,8,10,12:month:=31;
    4,6,9,11:month:=30;
    2:begin if year(nian)=365 then month:=28
            else month:=29;
      end;
  end;
end;

begin
  readln(y,m,d);
  readln(y0,m0,d0);
  y:=y+19;
  for i:=1 to m-1 do
    p:=p+month(i,y);
  p:=p+d;
  for i:=1 to m0-1 do
    q:=q+month(i,y0);
  q:=q+d0;
  for i:=y0 to y-1 do
    s:=s+year(i);
  s:=s+p-q;
  writeln(s);
end.