C++ RFC3339 timestamp with milliseconds using std::chrono

Question

I'm creating an RFC3339 timestamp, including milliseconds and in UTC, in C++ using std::chrono like so:

#include <chrono>
#include <ctime>
#include <iomanip>

using namespace std;
using namespace std::chrono;

string now_rfc3339() {
  const auto now = system_clock::now();
  const auto millis = duration_cast<milliseconds>(now.time_since_epoch()).count() % 1000;
  const auto c_now = system_clock::to_time_t(now);

  stringstream ss;
  ss << put_time(gmtime(&c_now), "%FT%T") <<
    '.' << setfill('0') << setw(3) << millis << 'Z';
  return ss.str();
}

// output like 2019-01-23T10:18:32.079Z

(forgive the usings)

Is there a more straight forward way of getting the milliseconds of now? It seems somewhat cumbersome to %1000 the now in milliseconds to get there. Or any other comments on how to do this more idiomatic?

Atropos

Answer 1

You could also do this with subtraction:

string
now_rfc3339()
{
    const auto now_ms = time_point_cast<milliseconds>(system_clock::now());
    const auto now_s = time_point_cast<seconds>(now_ms);
    const auto millis = now_ms - now_s;
    const auto c_now = system_clock::to_time_t(now_s);

    stringstream ss;
    ss << put_time(gmtime(&c_now), "%FT%T")
       << '.' << setfill('0') << setw(3) << millis.count() << 'Z';
    return ss.str();
}

This avoids the "magic number" 1000.

Also, there is Howard Hinnant's free, open source, single-header, header-only datetime library:

string
now_rfc3339()
{
    return date::format("%FT%TZ", time_point_cast<milliseconds>(system_clock::now()));
}

This does the same thing but with easier syntax.

Paolapaolina

Thanks Howard, stumbled across your date lib already, it looks very helpful! I'll keep it in mind for the future. But in this project, there's only one place where I need that format, so I rather not include an external source for that. – Atropos