std::string date ()
{
  char date [10];
  char time[10];
  std::ostringstream os;
  _strdate(date);
  _strtime(time);
  os << date << " " << time << ".txt";
  return os.str();
}
const std::string PATH = "e:\\" + date();
std::ofstream out(PATH.c_str(), std::ios::out);

#include <iostream>
#include <fstream>
#include <string>
#include <time.h>

using namespace std;

std::string
mkname ()
{
  char buf[80];
  time_t ltime = time (NULL);
  struct tm *today = localtime (&ltime);
  strftime (buf, sizeof (buf), "%y%m%d%H%M", today);
  std::string s = std::string(buf) + std::string(".txt");
  return s;
}

int
main (int argc, char *argv[])
{
  std::string sPath = mkname ();
  std::ofstream out;
  out.open(sPath.c_str(), std::ios::out);
  out << "Hello world!\n";
  return 0;
}