为什么我的vc++2015时间函数mktime总是返回-1？

代码如下，不论输入什么时间，比如2018-12-30，mktime总是得到-1，很是奇怪，请教高手这是怎么回事？
#include <iostream>
#include <ctime>

int main(int argc, char *argv[])
{
struct tm time0 = { 0 }, time1 = { 0 };
int year, mon, mday;
time0.tm_year = 70; //mktime能接受的最小年数
time0.tm_mon = 0; //1月
time0.tm_mday = 1; //1日
time0.tm_hour = 8; //8时 +8时区在1970年所能接受的最小小时数
time0.tm_hour = 0;
time0.tm_min = 0;
time0.tm_sec = 0;
printf(""); scanf("%4d-%2d-%2d", &year, &mon, &mday);
time1.tm_year = year - 1900; //struct tm里面，tm_year以1900年为基准。
time1.tm_mon = mon - 1;
time1.tm_mday = mday;
time1.tm_hour = 0;
time1.tm_min = 0;
time1.tm_sec = 0;
auto t1 = mktime(&time1);
auto t2 = mktime(&time0);
long seconds = (long)(difftime(t1, t2));
std::cout << seconds << std::endl;
}

 #include <iostream>
#include <ctime>

int main(int argc, char *argv[])
{
tm time0, time1;
int year, mon, mday;
time0.tm_year = 70; //mktime能接受的最小年数 
time0.tm_mon = 0; //1月 
time0.tm_mday = 1; //1日 
time0.tm_hour = 8; //8时 +8时区在1970年所能接受的最小小时数 
//time0.tm_hour = 0;
time0.tm_min = 0;
time0.tm_sec = 0;
printf(""); scanf("%4d-%2d-%2d", &year, &mon, &mday);
time1.tm_year = year - 1900; //struct tm里面，tm_year以1900年为基准。
time1.tm_mon = mon - 1;
time1.tm_mday = mday;
time1.tm_hour = 0;
time1.tm_min = 0;
time1.tm_sec = 0;
auto t1 = mktime(&time1);
auto t2 = mktime(&time0);
long seconds = (long)(difftime(t1, t2));
std::cout << seconds << std::endl;
}

2018-12-30
1546099200
Press any key to continue . . .

1，注释time0.tm_hour = 0;一句，结果t2=0；
2，time0.tm_mon = 0改为1，t2=2678400 __int64
3，我输入的1972-5-29，输出为73238400

你的tm_hour连续赋值两次，第二次为0，mktime的结果就是-1了