Question

I’ve written a function to convert datetimes from one format to another –

int convertDateTime(char* source_fmt,char* dest_fmt,char* source,char* dest)
{
        struct tm tmpptr;
        if (strptime(source,source_fmt,&tmpptr) == NULL)
        {
                strcpy(dest,"");
                return -1;
        }
        strftime(dest,100,dest_fmt,&tmpptr);
        return 0;
}

It works fine for most formats, But when I use format = “%y%j”, all I get is 10001; the julian day does not work.

I’m using gcc on solaris 10. Any idea what i need to change?

Answer 1

Update
The original answer (below) presumed that the “%y%j” format was used on the output (strftime) not the input (strptime). The mktime function will compute yday from valid info, but it doesn’t work the other way.

If you want to decode something like that you need to do it manually. Some example code follows. It has had minimal testing and almost certainly has bugs. You may need to alter ir, depending in the input format you want.

#include <string>
#include <ctime>
#include <cassert>
#include <iostream>

int GetCurrentYear()
{
    time_t tNow(::time(NULL));
    struct tm tmBuff = *::localtime(&tNow);
    return tmBuff.tm_year;
}
bool IsLeapYear(int nYear)
{
    if (0 == (nYear%1000)) return true;
    if (0 == (nYear%100))  return false;
    if (0 == (nYear%4))    return true;
    return false;
}
// nMonth = 0 (Jan) to 11 (Dec)
int DaysPerMonth(int nMonth, bool bLeapYear)
{
    //                 J   F   M   A   M   J   J   A   S   O   N   D
    int nDays[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
    assert(nMonth>=0 && nMonth<12);
    int nRet = nDays[nMonth];
    if (bLeapYear && nMonth==1)
        nRet++;
    return nRet;
}

// sDate is in the format YYDDD where YY is the last 2 digits of the year
// and YYY is the day of the year (1/1 = 1, 31/12 = 365 for non-leap year)
bool DecodeDate(const std::string &sDate, struct tm &tmBuff)
{
    if (sDate.length() != 5 ||
        !isdigit(sDate[0])  ||
        !isdigit(sDate[1])  ||
        !isdigit(sDate[2])  ||
        !isdigit(sDate[3])  ||
        !isdigit(sDate[4]))
    {
        return false;
    }
    ::memset(&tmBuff, 0, sizeof(struct tm));
    tmBuff.tm_year = GetCurrentYear();
    // drop last 2 digits
    tmBuff.tm_year -= tmBuff.tm_year%100; 
    // replace last 2 digits
    tmBuff.tm_year += ::atoi(sDate.substr(0, 2).c_str());    

    tmBuff.tm_yday = ::atoi(sDate.substr(2).c_str());
    int nDays(tmBuff.tm_yday);
    bool bLeapYear(IsLeapYear(1900 + tmBuff.tm_year));
    int nTmp = DaysPerMonth(0, bLeapYear);
    while (nTmp < nDays)
    {
        nDays -= nTmp;
        tmBuff.tm_mon++;
        nTmp = DaysPerMonth(tmBuff.tm_mon, bLeapYear);
    }
    tmBuff.tm_mday = nDays;
    ::mktime(&tmBuff);
    return true;
}

int main(int argc, char *argv[])
{
    for (int i=1; i<argc; i++)
    {
        struct tm tmBuff;
        DecodeDate(argv[i], tmBuff);
        const size_t nSize(128);
        char szBuff[nSize];
        strftime(szBuff, nSize, "%A, %d %B %Y", &tmBuff);
        std::cout << argv[i] << '\t' << szBuff << std::endl;
    }
    return 0;
}

================================================

C:\Dvl\Tmp>Test.exe 07123 08123 08124
07123   Thursday, 03 May 2007
08123   Friday, 02 May 2008
08124   Saturday, 03 May 2008

End Update

After you call strptime(), call mktime() which will populate any missing members of the struct. Also, you should zero out the struct before beginning.

#include <string>
#include <ctime>

int convertDateTime(const std::string &sSourceFmt, 
                    const std::string &sDestFmt,
                    const std::string &sSource,
                    std::string &sDest)
{
    struct tm tmbuff = { 0 };

    if (::strptime(sSource.c_str(), sSourceFmt.c_str(), &tmbuff) != NULL)
    {
        ::mktime(&tmbuff);
        const size_t nSize(256);
        char szBuff[nSize+1] = "";

        if (::strftime(szBuff, nSize, sDestFmt.c_str(), &tmbuff))
        {
            sDest = szBuff;
            return 0;
        }
    }
    sDest.clear();
    return -1;
}