Strange behavior with CDateTimeCtrl

I'm using MSVC++ 6 in Windows XP Pro SP2, and trying to write a simple dialog with a date/time picker control, and am experiencing some strange behavior; when I try to UpdateData() after entering a time in the control, the last assertion in the following CTime constructor fails, even though all input fields appear valid:

Code:

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CTime::CTime(int nYear, int nMonth, int nDay, int nHour, int nMin, int nSec, int nDST)

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In particular, this assertion fails for any time earlier than 6 PM. I suspected that my time zone setting might be influencing the problem, so I set my time zone to GMT, and indeed the error vanished.
Obviously I can't insist my users set their time zone to GMT before running my program - is there something obvious I've missed? Are my MFC source files simply outdated, and if so, is a new version freely available? Thanks for your help.

Can you post the constructor code up to the assertion that failed and your call of said constructor please ?

Certainly.

The constructor code:

Code:

---

CTime::CTime(int nYear, int nMonth, int nDay, int nHour, int nMin, int nSec, int nDST)
{
struct tm atm;
atm.tm_sec = nSec;
atm.tm_min = nMin;
atm.tm_hour = nHour;
ASSERT(nDay >= 1 && nDay <= 31);
atm.tm_mday = nDay;
ASSERT(nMonth >= 1 && nMonth <= 12);
atm.tm_mon = nMonth -1; // tm_mon is 0 based
ASSERT(nYear >= 1900);
atm.tm_year = nYear - 1900; //tm_year is 1900 based
atm.tm_isdst = nDST;
m_time = mktime(&atm);
ASSERT(m_time != -1); //This assertion fails
}

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The above is called from the following other CTime constructor:

Code:

---

CTime::CTime(const SYSTEMTIME& sysTime, int nDST)
{
if(sysTime.wYear < 1900)
{
    time_t time0 = 0L;
    CTime timeT(time0);
    *this = timeT;
}
else
{
    //This next line calls the constructor with failed assertion
    CTime timeT( (int)sysTime.wYear, (int)sysTime.wMonth, (int)sysTime.wDay, (int)sysTime.wHour, (int)sysTime.wMinute, (int)sysTime.wSecond, nDST);
    *this = timeT;
}
}

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which in turn is called by

Code:

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DWORD CDateTimeCtrl::GetTime(CTime& timeDest) const
{
    SYSTEMTIME sysTime;
    DWORD dwResult = (DWORD)
              ::SendMessage(m_hWnd, DTM_GETSYSTEMTIME, 0, (LPARAM) &sysTime);

    if (dwResult == GDT_VALID)
        timeDest = CTime(sysTime); //Execution flow goes through here

    return dwResult;
}

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Note that all of the above is stock MFC code; the only thing I do is call UpdateData(), which calls DoDataExchange, which calls the above.