Displaying Dates

Displaying Dates

We haven't discussed time series yet, when do, we will need to describe dates. There are countless ways to format a date but there is only one way we will do it, ISO8601 1. In the ISO8601 we represent dates like so:

2006-01-02T15:04:05Z07:00

This corresponds to:

Mon Jan 2 15:04:05 MST 2006

Displaying Dates

strftime

The more common way to format time is strftime. This approach uses a collection of symbols to represent components of time, for a list of these symbols see man strftime 2, R's ?strftime and Python's help(time.strftime):

man strftime

The characters of ordinary character sequences (including the null byte) are copied verbatim from format to s.  However, the characters of conversion specifications are replaced as shown in
    the list below.  In this list, the field(s) employed from the tm structure are also shown.

    `%a`     The abbreviated name of the day of the week according to the current locale.  (Calculated from tm_wday.)  (The specific names used in the current locale can be  obtained  by  calling
           nl_langinfo(3) with ABDAY_{1–7} as an argument.)

    `%A`     The  full name of the day of the week according to the current locale.  (Calculated from tm_wday.)  (The specific names used in the current locale can be obtained by calling nl_lang‐
           info(3) with DAY_{1–7} as an argument.)

    `%b`     The abbreviated month name according to the current locale.  (Calculated from tm_mon.)  (The specific names used in the current locale can be obtained by calling nl_langinfo(3)  with
           ABMON_{1–12} as an argument.)

    `%B`     The  full  month  name  according  to  the  current  locale.  (Calculated from tm_mon.)  (The specific names used in the current locale can be obtained by calling nl_langinfo(3) with
              MON_{1–12} as an argument.)

The strftime approach was named after the C library that popularized it, it's used in C++, Rust, R, Python, PHP, Ruby and many more.

To represent in ISO8601 via strftime, the format would be %Y-%m-%dT%H:%M:%S, so e.g.:

  • R

    format(Sys.time(), "%Y-%m-%dT%H:%M:%S")

  • Python

    from datetime import datetime

now = datetime.now()
formatted = now.strftime("%Y-%m-%dT%H:%M:%S")
print(formatted)

  • Shell

    # ISO8601
date "+%Y-%m-%dT%H:%M:%S"
# Unix
date %+s
More Languages

  • Rust

    extern crate chrono;

use chrono::offset::Local;
use chrono::DateTime;

fn main() {
    let now: DateTime<Local> = Local::now();
    
    println!("{}", now.format("%Y-%m-%dT%H:%M:%S").to_string());
}

  • Julia

    using Dates

println(Dates.format(now(), "yyyy-mm-ddTHH:MM:SS"))

  • C

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

int main() {
char buffer[80];
time_t t = time(NULL);
struct tm *tm_info;

tm_info = localtime(&t);
strftime(buffer, 80, "%Y-%m-%dT%H:%M:%S", tm_info);

printf("%s\n", buffer);

return 0;
}

You'll notice that each language (except Go and Julia) uses a similar approach to format the time, standardized by the strftime approach.

Reference Time

In this approach the language requires a specific date to format time, the specific date (Mon Jan 2 15:04:05 MST 2006) was a design decision made by the development team because they are distinct in the context of a date -- they used each number from 0 to 6 only once:

  • 2006: Represents the full year format.
  • 01: Stands for the numerical month format.
  • 02: Represents the day of the month.
  • 15: Stands for the 24-hour format time.
  • 04: Represents the minutes.
  • 05: Represents the seconds.
  • MST: Time zone
  • Mon and Jan: Are used for the abbreviated day of the week and month respectively.

Reference time is not a common approach, it's more of a unique quirk of the Go language. We draw attention to it here because it is a convenient way to express time in a human readable way, this can be useful when looking up documentaion, researching or querying a LLM. For example, to print the current time in ISO8601 format we would write it like this:

package main

import (
	"fmt"
	"time"
)

func main() {
	t := time.Now()
	fmt.Println(t.Format("2006-01-02T15:04:05-0700"))
}