Outputting date in ISO 8601 format

Question

How can I get a date in the following format in C++:

2016-04-26T19:50:48Z

#include <chrono>
#include <ctime>

time_t _tm = time(NULL);
struct tm*curtime = localtime(&_tm);

And outputting as asctime(curtime)

The current output is: "Thu Apr 28 16:02:41 2016\n"

http://en.cppreference.com/w/cpp/chrono/c/strftime – Michael Albers Apr 29 '16 at 00:35 — Michael Albers, Apr 29 '16 at 00:35
No research effort in evidence = downvote – Lightness Races in Orbit Apr 29 '16 at 01:02 — Lightness Races in Orbit, Apr 29 '16 at 01:02
https://stackoverflow.com/a/26896792/1599699 – Andrew Aug 25 '18 at 18:52 — Andrew, Aug 25 '18 at 18:52

Baum mit Augen · Accepted Answer · 2018-05-11T18:05:42.933

14

Documentation is your friend:

std::time_t t
    = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
std::cout << std::put_time( std::localtime( &t ), "%FT%T%z" );

in my system yields

2016-04-29T02:48:56+0200

edited May 11 '18 at 18:05

answered Apr 29 '16 at 00:50

Baum mit Augen

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Do you know how to display the millisecond part of the timestamp? – aviggiano Aug 03 '17 at 22:50
@aviggiano I don't think the millisecond part is part of the ISO 8601 format, so that would be a new question. – Baum mit Augen Oct 17 '17 at 09:31
1

I have to disagree (see https://en.wikipedia.org/wiki/ISO_8601#Times and especially the last paragraph there). Basically, you can put a decimal fraction part behind any of the time components, but it must be the last given. For millisecond precision it would be something like `2019-06-18T18:12:34.456` (or comma instead of decimal point), but the `put_time()` method doesn't seem to support a format specifier for the millisecond part. One solution that helped me, was this one: https://stackoverflow.com/a/35157784/5823275. – Chris Tophski Jun 18 '19 at 17:42

score 4 · Answer 2 · answered Feb 12 '20 at 18:39

Based on @Uri's answer which fixes a few bugs and shows the time in the proper timezone along with the milliseconds in ISO8601 format:

auto now = std::chrono::system_clock::now();
std::time_t time = std::chrono::system_clock::to_time_t(now);
std::tm* now_tm = std::localtime(&time);
long long timestamp = std::chrono::duration_cast<std::chrono::milliseconds>(now.time_since_epoch()).count();

std::cout << std::setfill('0') 
          << std::put_time(now_tm, "%FT%H:%M:")
          << std::setw(2) << (timestamp / 1000) % 60 << '.'
          << std::setw(3) << timestamp % 1000
          << std::put_time(now_tm, "%z");

Note that std::localtime may not be thread-safe. – Emile Cormier Aug 28 '22 at 00:41 — Emile Cormier, Aug 28 '22 at 00:41

score 2 · Answer 3 · answered Feb 11 '20 at 15:49

I'm combining the std::localtime which gives me calendar values, with std::chrono functions that gives me the precise methods. Here is my code:

#include <ctime>
#include <chrono>

...

auto now = std::chrono::system_clock::now();
auto now_c = std::chrono::system_clock::to_time_t(now)
auto now_tm = std::localtime(&now_c);
auto now_since_epoch = now.time_since_epoch();    // since 1970
auto now_ms = std::chrono::duration_cast<std::chrono::milliseconds>(now_since_epoch).count();

std::cout << std::setfill('0') <<
    std::setw(4) << now_tm->tm_year + 1900 << '-' <<
    std::setw(2) << now_tm->tm_mon + 1 << '-' <<
    std::setw(2) << now_tm->tm_mday << 'T' <<
    std::setw(2) << now_ms % (24*60*60*1000) << ':' <<
    std::setw(2) << now_ms % (60*60*1000) << ':' <<
    std::setw(2) << now_ms % (60*1000) << '.' <<
    std::setw(3) << now_ms % (1000);

Although verbose, it is actually doing less than strftime.

Outputting date in ISO 8601 format

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