Boost ptime: how to format data in a way browsers send inside headers of http requests?

I need to format my ptime in such way Wed, 21 Jan 2004 19:51:30 GMT How to do such thing with boost? (so it would look like data format of HTTP servers Expires and Last-Modified 开发者_StackOverflow中文版and Date response headers )

#include <locale>
#include <string>
#include <iostream>
#include <sstream>
#include <boost/date_time/posix_time/posix_time.hpp>

std::string current_time_formatted()
{
    namespace bpt = boost::posix_time;

    static char const* const fmt = "%a, %d %b %Y %H:%M:%S GMT";
    std::ostringstream ss;
    // assumes std::cout's locale has been set appropriately for the entire app
    ss.imbue(std::locale(std::cout.getloc(), new bpt::time_facet(fmt)));
    ss << bpt::second_clock::universal_time();
    return ss.str();
}

See Date Time Input/Output for more information on the available format flags.