C++: format time in ISO-8601

#include <chrono>

std::string to_iso_8601(std::chrono::time_point<std::chrono::system_clock> t)
{
    // convert to time_t which will represent the number of
    // seconds since the UNIX epoch, UTC 00:00:00 Thursday, 1st. January 1970
    time_t epoch_seconds = std::chrono::system_clock::to_time_t(t);

    // Format this as date time to seconds resolution
    // e.g. 2016-08-30T08:18:51
    std::stringstream stream;
    struct tm buf;
    gmtime_s(&buf, &epoch_seconds);
    stream << std::put_time(&buf, "%FT%T");

    // If we now convert back to a time_point we will get the time truncated
    // to whole seconds 
    auto truncated = std::chrono::system_clock::from_time_t(epoch_seconds);

    // Now we subtract this seconds count from the original time to
    // get the number of extra microseconds..
    auto delta_us = std::chrono::duration_cast<std::chrono::microseconds>(t - truncated).count();

    // And append this to the output stream as fractional seconds
    // e.g. 2016-08-30T08:18:51.867479
    stream << "." << std::fixed << std::setw(6) << std::setfill('0') << delta_us;

    return stream.str();
}

std::string to_iso_8601(std::chrono::time_point<std::chrono::system_clock> t = std::chrono::system_clock::now());