Demo date time format using UTC ISO

This demo shows:

How to print a date-time string.
Using the UTC time zone, also known as +00:00, or GMT, or Zulu time.
Using the ISO 8601 extended format, because it's easy to read.

Example:

2020-01-01T00:00:00.000+00:00

Meaning:

YYYY-MM-DD means the year, month, and day.
T is the ISO standard separator character between the date and time.
HH:MM:SS.sss means the hour, minute, second, and millisecond.
+00:00 means zero offset from UTC, in other words, actual UTC time.

Preferences:

We like the extended format because it's easy for a person to skim.
We prefer using a T separator over a blank because of machine parsing.
We prefer using fractional seconds over just seconds because of precision.
We prefer +00.00 over Z because our logs contain many time zones.

Coding conventions in this repo:

t is the time.
f is the format string.
s is the output string.

Examples:

Bash
C
C++
Elixir
Go
Java
Python
Ruby

Bash

Bash shell with GNU date and nanoseconds:

date -u +"%Y-%m-%dT%H:%M:%S.%N+00:00"

Bash shell with BSD date and seconds:

date -u +"%Y-%m-%dT%H:%M:%S+00:00"

Bash shell with BSD date to convert Unix epoch seconds:

date -r 1000000000 -u +"%Y-%m-%dT%H:%M:%S+00:00"

Bash shell with BSD date to find files and print times:

find . -type f -print0 | 
xargs -0 stat -f"%m␟%N" |
awk -F ␟ '{ ("date -r " $1 " -u +\"%Y-%m-%dT%H:%M:%S+00:00\"" | getline t); $1=t; print}' |
sort -n

C

C with ANSI C:

#include <stdio.h>
#include <time.h>

int main()
{
    time_t timer;
    char s[30];
    struct tm* tm_info;

    time(&timer);
    tm_info = localtime(&timer);

    strftime(s, 30, "%Y-%m-%d %H:%M:%S.000000000+00:00", tm_info);
    puts(s);

    return 0;
}

C with struct timeval:

#include <stdio.h>
#include <sys/time.h>
int main(void)
{
    struct timeval t;
    gettimeofday(&t,NULL);
    printf("%ld.%09ld+00:00\n", (long int)t.tv_sec, (long int)t.tv_usec);
    return 0;
}

C++

C++:

#include <iostream>

int main() {
    time_t t;
    time(&t);
    char buf[sizeof "2011-10-08T07:07:09+00:00"];
    strftime(buf, sizeof buf, "%Y-%m-%dT%H:%M:%S+00:00", gmtime(&t));
    // Prefer this line if your compiler supports %F or %T formats:
    //strftime(buf, sizeof buf, "%FT%TZ", gmtime(&nt));
    std::cout << buf << "\n";
}

C++ with 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";
}

Elixir

Elixir with the Timex library:

use Timex
f = "{ISO:Extended}"
t = Timex.now 
Timex.format(t, f)

Go

Go:

package main

import "fmt"
import "time"

func main() {
    const f = "2006-01-02T15:04:05.999999999+00:00"
    t := time.Now().UTC()
    s := t.Format(format)
    fmt.Println(s)
}

Java

Java with seconds:

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class DateTimeFormat {
  public static void main(String[] args) {
     String iso = "yyyy-MM-dd'T'HH:mm:ss'.000000000+00:00'";
     TimeZone tz = TimeZone.getTimeZone("UTC");
     DateFormat df = new SimpleDateFormat(iso);
     df.setTimeZone(tz);
     String s = df.format(new Date());
     System.out.println(s);
  }
}

Java with Joda:

DateTime dt = new DateTime();
DateTimeFormatter iso = ISODateTimeFormat.dateTime();
String str = fmt.print(dt);

JavaScript

JavaScript with milliseconds:

var t = new Date();
var s = now.toISOString().slice(0, -1) + "+00:00"
console.log(s);

Perl

Perl with POSIX and seconds:

use POSIX;
my $F = "%Y-%m-%dT%H:%M:%S+00:00"
my $t = time();
print strftime($F, gmtime($t), "\n";

Perl with CPAN:

use DateTime;
my $t = DateTime->now()
$now->iso8601().'+00:00';

Python

Python with microseconds:

import datetime
F = "%Y-%m-%dT%H:%M:%S.%f+00:00" 
t = datetime.datetime.utcnow()
t.strftime(F)

Ruby

Ruby:

F = "%Y-%m-%dT%H:%M:%S.%N+00:00"
t = Time.now.utc
puts t.strftime(F)

joelparkerhenderson / demo-date-time-format