请教各位C++大神，看到请进！字符串转时间类型。

#include <stdio.h>
#include <time.h>
char a[]="19900101101010";
char b[]="20151118131023";
struct tm sta;
struct tm stb;
time_t ta;
time_t tb;
char tmpbuf[128];
int main() {
    sscanf(a,"%4d%2d%2d%2d%2d%2d%2d",&sta.tm_year,&sta.tm_mon,&sta.tm_mday,&sta.tm_hour,&sta.tm_min,&sta.tm_sec);sta.tm_year-=1900;sta.tm_mon--;
    sscanf(b,"%4d%2d%2d%2d%2d%2d%2d",&stb.tm_year,&stb.tm_mon,&stb.tm_mday,&stb.tm_hour,&stb.tm_min,&stb.tm_sec);stb.tm_year-=1900;stb.tm_mon--;
    ta=mktime(&sta);
    tb=mktime(&stb);
    printf("From\n");
    strftime(tmpbuf,128,"%Y-%m-%d %H:%M:%S",localtime(&ta));printf("%s\n",tmpbuf);
    printf("to\n");
    strftime(tmpbuf,128,"%Y-%m-%d %H:%M:%S",localtime(&tb));printf("%s\n",tmpbuf);
    printf("elapse\n");
    printf("%.15lg\n",difftime(tb,ta));
    printf("second(s).\n");
    return 0;
}
//From
//1990-01-01 10:10:10
//to
//2015-11-18 13:10:23
//elapse
//816663613
//second(s).
//