C++11新特性，std::chrono获取系统时间

const std::string getCurrentSystemTime()
{
  auto tt = std::chrono::system_clock::to_time_t
  (std::chrono::system_clock::now());
  struct tm* ptm = localtime(&tt);
  char date[60] = {0};
  sprintf(date, "%d-%02d-%02d      %02d:%02d:%02d",
    (int)ptm->tm_year + 1900,(int)ptm->tm_mon + 1,(int)ptm->tm_mday,
    (int)ptm->tm_hour,(int)ptm->tm_min,(int)ptm->tm_sec);
  return std::string(date);
}