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

Question

I\'m working with the Azure REST API and they are using this to create the request body for table storage:

DateTime.UtcNow.ToString(\"o\")

Answer 1

Tested in Visual C++, GNU C++, Emscripten

#include <ctime>
#include <chrono>
#include <iostream> 
#include <locale>  

#if defined (_WIN32) 
#define WINDOWSLIB 1
#elif defined (__APPLE__)//iOS, Mac OS
#define MACOSLIB 1
#elif defined (__LINUX__) || defined(__gnu_linux__) || defined(__linux__) || defined(__linux) || defined(linux)//_Ubuntu - Fedora - Centos - RedHat
#define LINUXLIB 1
#elif defined (__EMSCRIPTEN__)
#define EMSCRIPTENLIB 1
#endif

#define WriteLine(data)std::cout<< data <<std::endl;
typedef std::string String;

String CurrentISO8601DateTime(bool toUTC=true)
{
    using namespace std::chrono;
    system_clock::time_point now = system_clock::now();
    time_t timet = system_clock::to_time_t(now);
    std::tm tm{};
    String localeStr = setlocale(LC_ALL, nullptr);
    setlocale(LC_ALL, u8"");
    String format = String(u8"%FT%T.").append(std::to_string(duration_cast<milliseconds>(now.time_since_epoch()).count() % static_cast<long long>(1000)));
    if (toUTC)
    {
#ifdef WINDOWSLIB
        gmtime_s(&tm, &timet);
#elif LINUXLIB
        gmtime_r(&timet, &tm);
#elif EMSCRIPTENLIB
        gmtime_r(&timet, &tm);
#endif
        format = format.append(u8"Z");
    }
    else
    {
#ifdef WINDOWSLIB
        localtime_s(&tm, &timet);
#elif LINUXLIB
        localtime_r(&timet, &tm);
#elif EMSCRIPTENLIB
        localtime_r(&timet, &tm);
#endif
        format.append(u8"%z");
    }
    String result = String(255, 0);
    const size_t length = std::strftime(&result[0], result.size(), format.c_str(), &tm);
    result.resize(length);
    setlocale(LC_ALL, localeStr.c_str());
    return result;
}

#define ConsoleWriteLn(data) std::cout<< data <<std::endl;

int main()
{
    ConsoleWriteLn(u8"UTC  : " + CurrentISO8601DateTime());
    ConsoleWriteLn(u8"LOCAL: " + CurrentISO8601DateTime(false));
}

---

Results

UTC : 2020-04-12T17:00:18.632Z
LOCAL: 2020-04-12T12:00:18.633-0500

You can deserialize normally with Json.NET

Answer 2

In Qt, that would be:

QDateTime dt = QDateTime::currentDateTime();
dt.setTimeSpec(Qt::UTC);  // or Qt::OffsetFromUTC for offset from UTC
qDebug() << QDateTime::currentDateTime().toString(Qt::ISODate);

Answer 3

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"];
    strftime(buf, sizeof buf, "%FT%T", gmtime(&curTime.tv_sec));
    sprintf(buf, "%s.%dZ", buf, milli);

    return buf;
}

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