How do I construct an ISO 8601 datetime in C++?

Question

If the time to the nearest second is precise enough, you can use strftime:

#include <ctime>
#include <iostream>

int main() {
    time_t now;
    time(&now);
    char buf[sizeof "2011-10-08T07:07:09Z"];
    strftime(buf, sizeof buf, "%FT%TZ", gmtime(&now));
    // this will work too, if your compiler doesn't support %F or %T:
    //strftime(buf, sizeof buf, "%Y-%m-%dT%H:%M:%SZ", gmtime(&now));
    std::cout << buf << "\n";
}

If you need more precision, you can use Boost:

#include <iostream>
#include <boost/date_time/posix_time/posix_time.hpp>

int main() {
    using namespace boost::posix_time;
    ptime t = microsec_clock::universal_time();
    std::cout << to_iso_extended_string(t) << "Z\n";
}

Using the date library (C++11):

template <class Precision>
string getISOCurrentTimestamp()
{
    auto now = chrono::system_clock::now();
    return date::format("%FT%TZ", date::floor<Precision>(now));
}

Example usage:

cout << getISOCurrentTimestamp<chrono::seconds>();
cout << getISOCurrentTimestamp<chrono::milliseconds>();
cout << getISOCurrentTimestamp<chrono::microseconds>();

Output:

2017-04-28T15:07:37Z
2017-04-28T15:07:37.035Z
2017-04-28T15:07:37.035332Z

I should point out I am a C++ newb.

I needed string with a UTC ISO 8601 formatted date and time that included milliseconds. I did not have access to boost.

This is more of a hack than a solution, but it worked well enough for me.

std::string getTime()
{
    timeval curTime;
    gettimeofday(&curTime, NULL);

    int milli = curTime.tv_usec / 1000;
    char buf[sizeof "2011-10-08T07:07:09.000Z"];
    char *p = buf + strftime(buf, sizeof buf, "%FT%T", gmtime(&curTime.tv_sec));
    sprintf(p, ".%dZ", milli);

    return buf;
}

The output looks like: 2016-04-13T06:53:15.485Z

Boost has a library for this.

I.e. posix_time has the from_iso_string() and to_iso_string() functions.

With C++20, time point formatting (to string) is available in the (chrono) standard library. https://en.cppreference.com/w/cpp/chrono/system_clock/formatter

#include <chrono>
#include <format>
#include <iostream>

int main()
{
   const auto now = std::chrono::system_clock::now();
   std::cout << std::format("{:%FT%TZ}", now) << '\n';
}

Output

2021-11-02T15:12:46.0173346Z

It works in Visual Studio 2019 with the latest C++ language version (/std:c++latest).