SYSTEMTIME 与 time_t 之间的转换，计算2个SYSTEMTIME的时间差

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time_t systemtime_to_time_t(const SYSTEMTIME& st)
{
    struct tm gm = {st.wSecond, st.wMinute, st.wHour, st.wDay, st.wMonth-1, st.wYear-1900, st.wDayOfWeek, 0, 0};
    return mktime(&gm);
}

SYSTEMTIME time_t_to_systemtime(time_t t)
{
    tm temptm = *localtime(&t);
    SYSTEMTIME st = {1900 + temptm.tm_year,
        1 + temptm.tm_mon,
        temptm.tm_wday,
        temptm.tm_mday,
        temptm.tm_hour,
        temptm.tm_min,
        temptm.tm_sec,
        0};
    return st;
}

https://blog.csdn.net/hellokandy/article/details/51329446

MFC版本的计算两个SYSTEMTIME之间的时间差，返回毫秒：

__int64 TimeDiff(SYSTEMTIME left, SYSTEMTIME right)
{
    CTime tmLeft(left.wYear, left.wMonth, left.wDay, 0, 0, 0);
    CTime tmRight(right.wYear, right.wMonth, right.wDay, 0, 0, 0);

    CTimeSpan sp = tmLeft - tmRight;
    long MillisecondsL = (left.wHour*3600 + left.wMinute*60 + left.wSecond)*1000 + left.wMilliseconds;
    long MillisecondsR = (right.wHour*3600 + right.wMinute*60 + right.wSecond)*1000 + right.wMilliseconds;

    return  (__int64)sp.GetDays()*86400000 + (MillisecondsL - MillisecondsR);//此处返回毫秒
}

https://blog.csdn.net/hellokandy/article/details/51329183