Graham Graham - 1 month ago 15
C++ Question

C++ Time returned is two hours out

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

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.

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