time_t curdate = time(0);
struct tm * tm_curdate = gmtime(&curdate);
char date_buf[11];
strftime(date_buf, 11, "%Y-%m-%d", tm_curdate);

std::stringstream buf;
buf << "SELECT ..."
    " WHERE my_date >= DATE_SUB(" << date_buf << ", INTERVAL 28 DAY)" <<
    " LIMIT 1";