std::chrono::steady_clock to ISO8601

std_chrono_steady_clock_to_ISO8601.cxx

auto to_string_iso8601( const std::chrono::steady_clock::time_point& t )
  -> std::string
{
  using namespace std::chrono;
  const time_t ct
  ( ( duration_cast< seconds >( system_clock::now( ).time_since_epoch( ) )
    + duration_cast< seconds >( t - steady_clock::now( ) )
    ).count( )
  );
  std::string r = "0000-00-00T00:00:00Z.";
  std::strftime( const_cast< char* >( r.data( ) ), r.size( ), "%FT%TZ", std::gmtime( &ct ) );
  return r;
}