これどういうことですかね…

/* g++ 4.7.2 */
#include <iostream>
#include <chrono>
#include <unistd.h>
int main()
{
std::chrono::system_clock::time_point tp1;
std::chrono::system_clock::time_point tp2;

tp1 = std::chrono::system_clock::now();
sleep( 3 );
tp2 = std::chrono::system_clock::now();

std::chrono::milliseconds m1 = std::chrono::duration_cast<std::chrono::milliseconds>( tp2 - tp1 );
std::cout << m1.count () << " msec." << std::endl;

std::time_t tt1, tt2;
tt1 = std::chrono::system_clock::to_time_t( tp1 );
tt2 = std::chrono::system_clock::to_time_t( tp2 );
std::cout << std::ctime( &tt1 )
<< std::ctime( &tt2 )
<< std::endl;
return 0;
}
// 3000 msec.
// Fri Apr 18 00:05:57 2014
// Fri Apr 18 00:05:57 2014