C++ Time returned is two hours out

Question

Bit of a c++ newbie so here we go;

I have a method that is parsing a date/time, however that date/time is passed to me always with 00:00:00 as the hh:mm:ss. As such i want to add in the values of the current systime in place of those values. I have methods that do this, and the first method is returning the correct time in UTC format.

bool CTRHTranslationRTNS::ParseDateSysTime(const char* pszString, time_t& tValue)
{
    ASSERT(pszString != NULL);

    // DateTime fields.
    enum { YEAR, MONTH, DAY, HOURS, MINS, SECS, NUM_FIELDS };

    CStringArray astrFields;

    // Split the string into the date and time fields.
    int nFields = CStringParser::Split(pszString, "- :T", astrFields);

    // Not DD/MM/YYYY HH:MM:SS format.
    if (nFields != NUM_FIELDS)
        return false;

    int anFields[NUM_FIELDS] = { 0 };

    // Parse field numbers.
    for (int i = 0; i < NUM_FIELDS; ++i)
        anFields[i] = atoi(astrFields[i]);

    tm oTime = { 0 };

        //Add System Time instead
        time_t sysyemTimeNow;
        struct tm * ptm;
        time ( &sysyemTimeNow );
        ptm = gmtime ( &sysyemTimeNow );

    // Copy fields to time struct.
    oTime.tm_mday  = anFields[DAY];
    oTime.tm_mon   = anFields[MONTH] - 1;
    oTime.tm_year  = anFields[YEAR] - 1900;
    oTime.tm_hour  = ptm->tm_hour;
    oTime.tm_min   = ptm->tm_min;
    oTime.tm_sec   = ptm->tm_sec;
    oTime.tm_isdst = -1;

    // Convert to time_t.
    tValue = mktime(&oTime);

    // Invalid field values.
    if (tValue < 0)
        return false;

    return true;
}

In the second method I do some formatting on the date/time and this results in 2 hours being removed from the time.

string CTRHTranslationRTNS::ConvertDateSysTimeToDateInUTC(const string& bossDate)
{
    time_t dealDate;
    if (ParseDateSysTime(bossDate.c_str(), dealDate))
    {
        struct tm * ptm = gmtime(&dealDate);
        char buffer [80];
        strftime(buffer,80,"%Y-%m-%d %H:%M:%S",ptm);
        return string(buffer);
    }
    else
    {
        throw exception(string("Invalid date/SysTime value: ").append(bossDate).c_str());   
    }   
}

Just to be clear, the ParseDateSysTime method returns the time with the correct UTC value of 11:53, but as soon as

struct tm * ptm = gmtime(&dealDate);

is called the time changes to 08:53. It suggests this is a product of calling the gmtime() method but i am not sure.

Many Thanks

Graham

why is there a two hour difference in the times when no time has been added. the UTC time should be 11:53 but its coming out 08:53 when gmtime is called in the second function — Graham, Oct 23 '12 at 12:02

score 4 · Accepted Answer · answered Oct 23 '12 at 12:09

4

The reson is the mktime() method used in the first function uses local time, but gmtime() uses UTC time.

See http://www.cplusplus.com/reference/clibrary/ctime/mktime/ and http://www.cplusplus.com/reference/clibrary/ctime/gmtime/ for further explanation.

answered Oct 23 '12 at 12:09

Hakan Serce

11,198
3
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Michael Haephrati · Answer 2 · 2016-10-10T11:16:49.240

2

Try this function:

CTime Time2UTC(CTime original)
{
    CString Formatted = original.FormatGmt(L"%Y%m%d%H%M%S");
    int Year, Month, Day, Hour, Minute;
    if (Formatted != L"" && Formatted.GetLength() >= 12)
    {
        Year = _wtol(Formatted.Left(4));
        Month = _wtol(Formatted.Mid(4, 2));
        Day = _wtol(Formatted.Mid(6,2));
        Hour = _wtol(Formatted.Mid(8, 2));
        Minute = _wtol(Formatted.Mid(10, 2));
        CTime result(Year, Month, Day, Hour, Minute, 0);
        return result;
    }
    else
        return (CTime)NULL;
}

edited Oct 10 '16 at 11:16

answered Jul 18 '15 at 10:07

Michael Haephrati

3,660
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Thanks for this! Saved me the hassle. I gotta say, it is SO annoying that date and time classes for major frameworks, like `CTime`, don't even BOTHER to build in functions to let you do things like obtain a UTC time broken down into components, or (in the case of another Microsoft date/time class, `COleDateTime`) construct a date/time object from a Posix timestamp, etc. Really pathetic. As though it'll be *unusual* for a programmer working with these objects to WANT to do arcane things such as doing it properly to handle time zones, or converting between one API and another. Who'd want THAT... – Dan Nissenbaum Oct 08 '16 at 00:26

C++ Time returned is two hours out

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