String representation of time_t?

Try std::stringstream.

#include <string>
#include <sstream>

std::stringstream ss;
ss << seconds;
std::string ts = ss.str();

A nice wrapper around the above technique is Boost’s lexical_cast:

#include <boost/lexical_cast.hpp>
#include <string>

std::string ts = boost::lexical_cast<std::string>(seconds);

And for questions like this, I’m fond of linking The String Formatters of Manor Farm by Herb Sutter.

UPDATE:

With C++11, use to_string().