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

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

Answer 2

OK so I've modified a few solutions that I've found as came up with the following :

static QString getTimeZoneOffset()
{
    QDateTime dt1 = QDateTime::currentDateTime();
    QDateTime dt2 = dt1.toUTC();
    dt1.setTimeSpec(Qt::UTC);

int offset = dt2.secsTo(dt1) / 3600;
if (offset >= 0)
    return QString("%1").arg(offset).rightJustified(2, '0',true).prepend("+");
return QString("%1").arg(offset).rightJustified(2, '0',true);
}

Then to easily format a date ( yyyy-MM-dd'T'HH:mm:ss.SSSZ ) :

static QString toISO8601ExtendedFormat(QDateTime date)
{
    QString dateAsString = date.toString(Qt::ISODate);
    QString timeOffset =  Define::getTimeZoneOffset();
    qDebug() << "dateAsString :" << dateAsString;
    qDebug() << "timeOffset :" << timeOffset;
    timeOffset = QString(".000%1%2").arg(timeOffset).arg("00");
    qDebug() << "timeOffset replaced :" << timeOffset;
    if(dateAsString.contains("Z",Qt::CaseInsensitive))
        dateAsString = dateAsString.replace("Z",timeOffset);
    else
        dateAsString = dateAsString.append(timeOffset);
        qDebug() << "dateAsString :" << dateAsString;
    return dateAsString;
}

For example GMT +2 would look like this : 2013-10-14T00:00:00.000+0200

Answer 3

Did it like this:

using namespace boost::posix_time;
ptime t = microsec_clock::universal_time();
qDebug() << QString::fromStdString(to_iso_extended_string(t) + "0Z"); // need 7 digits

Answer 4

You can use this function which uses std::put_time with a std::ostringstream to generate the resulting std::string.

#include <iostream>
#include <chrono>
#include <iomanip>
#include <sstream>
/**
 * Generate a UTC ISO8601-formatted timestamp
 * and return as std::string
 */
std::string currentISO8601TimeUTC() {
  auto now = std::chrono::system_clock::now();
  auto itt = std::chrono::system_clock::to_time_t(now);
  std::ostringstream ss;
  ss << std::put_time(gmtime(&itt), "%FT%TZ");
  return ss.str();
}
// Usage example
int main() {
    std::cout << currentISO8601TimeUTC() << std::endl;
}

Reference: https://techoverflow.net/2018/03/30/iso8601-utc-time-as-stdstring-using-c11-chrono/

Answer 5

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";
}

Answer 6

Boost has a library for this.

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