C++ Time returned is two hours out - c++

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

Answer 1

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.

Answer 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;
}