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Day.js tutorial

last modified July 7, 2020

Day.js tutorial shows how to work with date and time in JavaScript with Day.js module.

Day.js

Day.js is a minimalist JavaScript date library for parsing, validating, manipulating, and formatting dates. It is an alternative library to Moment.js and has largely compatible API. Moment.js is covered in Moment.js tutorial.

In this tutorial we work with Day.js in a Node application.

Setting up Day.js

First, we install Day.js. 

$ node -v
v11.5.0

We use Node version 11.5.0. 

$ npm init -y

We initiate a new Node application. 

$ npm i dayjs

We install Day.js with npm i dayjs command.

Day.js today's date

In the first example, we get today's date with Day.js.

now.js
const dayjs = require('dayjs');

let now = dayjs();

console.log(now.format());

The example prints current datetime. 

const dayjs = require('dayjs');

We load the Day.js library. 

let now = dayjs();

We get the current local datetime object with dayjs(). 

console.log(now.format());

We format the output with format(). By default, we get a long datetime format. 

$ node now.js
2019-05-14T10:12:54+02:00

The output is in the ISO standard format. The datetime parts are separated by T character. The string is ended with a time zone.

Creating Day.js objects

We can use several ways to create datetime Day.js objects. These objects are formatted to human-readable output.

create_objects.js
const dayjs = require('dayjs');

let d1 = dayjs("2018-06-03");
console.log(d1.format());

let d2 = dayjs.unix(1530471537);
console.log(d2.format());

let d3 = dayjs(new Date(2011, 11, 22));
console.log(d3.format());

The example creates date and time objects in three different ways. 

let d1 = dayjs("2018-06-03");

We create a dayjs object by parsing a string. 

let d2 = dayjs.unix(1530471537);
console.log(d2.format());

We use a unix time stamp (in seconds) to define a datetime object. 

let d3 = dayjs(new Date(2011, 11, 22));
console.log(d3.format());

Finally, we use a JavaScript built-in Date object to define a dayjs object. 

$ node create_objects.js
2018-06-03T00:00:00+02:00
2018-07-01T20:58:57+02:00
2011-12-22T00:00:00+01:00

This is the output.

Day.js formatting datetime

Day.js objects are formatted with the format() function.

formatting.js
const dayjs = require('dayjs');

let now = dayjs();

console.log("ISO")
console.log(now.format());

console.log("\nTime")
console.log(now.format("HH:mm:ss"));
console.log(now.format("h:mm:ss a"));

console.log("\nDate")
console.log(now.format("dddd, MMMM D YYYY"));
console.log(now.format("YYYY-MM-DD"));

The example formats date and time using Day's format() function. 

$ node formatting.js
ISO
2019-05-14T10:19:18+02:00

Time
10:19:18
10:19:18 am

Date
Tuesday, May 14 2019
2019-05-14

This is a sample output.

Day.js calculating datetime difference

With the diff() function, we can calculate the difference between two datetime objects.

difference.js
const dayjs = require('dayjs');

const date1 = dayjs("2019-14-05");
const date2 = dayjs("2018-06-25");

let df1 = date1.diff(date2); 
console.log(df1);

let df2 = date1.diff(date2, "month"); 
console.log(df2);

let df3 = date1.diff(date2, "month", true); 
console.log(df3);

let df4 = date1.diff(date2, "day"); 
console.log(df4);

let df5 = date1.diff(date2, "week"); 
console.log(df5);

The example calculates the difference between two dayjs objects in months, days, and weeks. 

let df2 = date1.diff(date2, "month"); 
console.log(df2);

The second parameter tells that the output will be in months. 

let df3 = date1.diff(date2, "month", true); 
console.log(df3);

Setting the third parameter to true returns a floating point value, e.g. 7.3 months. 

$ node difference.js
19357200000
7
7.3
224
32

This is the output.

The Battle of Borodino was a battle fought on 7 September 1812 in the Napoleonic Wars during the French invasion of Russia.

borodino.js
const dayjs = require('dayjs');

let borodinoBattle = dayjs('1812-09-07');

let now = dayjs();
let days = now.diff(borodinoBattle, 'days');

console.log(`On ${now.format('YYYY-MM-DD')}, ${days} days have passed since the Borodino battle.`);

In the example, we calculate the number of days passed since the famous battle. 

$ node borodino.js
On 2019-05-14, 75489 days have passed since the Borodino battle.

This is a sample output.

Day.js datetime arithmetic

The add() function is used to add date and time to the dayjs object and the subtract() function subtract date and time from the dayjs object.

arithm.js
const dayjs = require('dayjs');

let now = dayjs();

console.log(now.format('YYYY-MM-DD'));

let d1 = now.add('14', 'day');

console.log(d1.format('YYYY-MM-DD'));

let d2 = now.subtract('3', 'year');

console.log(d2.format('YYYY-MM-DD'));

In the example, we add fourteen days and subtract two years to and from the current datetime. 

let d1 = now.add('14', 'day');
...
let d2 = now.subtract('3', 'year'););

The second parameter of the add() and subtract() functions is the unit type. 

$ node arithm.js
2019-05-14
2019-05-28
2016-05-14

This is the output.

Day.js datetime parts

In the following example, we get the parts of the current datetime.

parts.js
const dayjs = require('dayjs');

let now = dayjs();

console.log(now.toObject());

let year = now.year();
console.log(`Year: ${year}`);

let month = now.month();
console.log(`Month: ${month}`);

let date = now.date();
console.log(`Date: ${date}`);

let hour = now.hour();
console.log(`Hour: ${hour}`);

let minute = now.minute();
console.log(`Minute: ${minute}`);

let second = now.second();
console.log(`Second: ${second}`);

let milli = now.millisecond();
console.log(`Millisecond: ${milli}`);

The example computes the current datetime. We get the year, month, date, hour, minute, second, and millisecond parts of the datetime. The toObject() function returns a JavaScript object with the datetime parts. 

$ node parts.js
{ years: 2019,
  months: 4,
  date: 14,
  hours: 10,
  minutes: 41,
  seconds: 34,
  milliseconds: 963 }
Year: 2019
Month: 4
Date: 14
Hour: 10
Minute: 41
Second: 34
Millisecond: 963

This is a sample output.

Day.js conversion functions

Apart from the format() function, we can also use a few built-in conversion functions.

converting.js
const dayjs = require('dayjs');

let now = dayjs();

console.log(now.toString());
console.log(now.toJSON());
console.log(now.toISOString());
console.log(now.toObject());

We have four functions. The toJSON() is an alias to toISOString(). 

$ node converting.js
Tue, 14 May 2019 09:00:51 GMT
2019-05-14T09:00:51.157Z
2019-05-14T09:00:51.157Z
{ years: 2019,
    months: 4,
    date: 14,
    hours: 11,
    minutes: 0,
    seconds: 51,
    milliseconds: 157 }

This is the output.

Day.js relative datetime

We can compute relative datetime with startOf() and endOf() functions.

relative_time.js
const dayjs = require('dayjs');

// let now = dayjs();

let startWeek = dayjs().startOf('week');
console.log(startWeek.format());

let endWeek = dayjs().endOf('week');
console.log(endWeek.format());

let startMonth = dayjs().startOf('month');
console.log(startMonth.format());

let endMonth = dayjs().endOf('month');
console.log(endMonth.format());

let startYear = dayjs().startOf('year');
console.log(startYear.format());

let endYear = dayjs().endOf('year');
console.log(endYear.format());

The example uses the aforementioned functions. 

let startWeek = dayjs().startOf('week');
console.log(startWeek.format());

Here we calculate the datetime of the start of the current week. 

let endYear = dayjs().endOf('year');
console.log(endYear.format());

Here we get the last datetime of the year. 

$ node relative_time.js
2019-05-12T00:00:00+02:00
2019-05-18T23:59:59+02:00
2019-05-01T00:00:00+02:00
2019-05-31T23:59:59+02:00
2019-01-01T00:00:00+01:00
2019-12-31T23:59:59+01:00

This is the output.

Day.js checking validity

We can use the isValid() function to check if the date and time object is valid.

validating.js
const dayjs = require('dayjs');

let day1 = dayjs('2018-12-12');
let day2 = dayjs('2018-11-ks');

if (day1.isValid()) {

    console.log("Day is valid");
    console.log(day1.format());
} else {
    
    console.log("Day is not valid");
}

if (day2.isValid()) {

    console.log("Day is valid");
    console.log(day2.format());
} else {
    
    console.log("Day is not valid");
}

The example checks the validity of two days.

Day.js date queries

The isSame(), isBefore(), and isAfter() functions can be used to determine if a date is before or after another date.

queries.js
const dayjs = require('dayjs');

let d1 = dayjs("2018-05-19");
let d2 = dayjs("2018-05-20");
let d3 = dayjs("2018-05-22");
let d4 = dayjs("2018-05-19");

if (d1.isSame(d4)) {

    console.log('these are same dates');
} else {

    console.log('these are not the same dates');
}

if (d1.isAfter(d2)) {

    console.log(`${d1.format('YYYY-MM-DD')} is after ${d2.format('YYYY-MM-DD')}`);
} else {

    console.log(`${d1.format('YYYY-MM-DD')} is before ${d2.format('YYYY-MM-DD')}`);
}

if (d2.isBefore(d3)) {

    console.log(`${d2.format('YYYY-MM-DD')} is before ${d3.format('YYYY-MM-DD')}`);
} else {

    console.log(`${d2.format('YYYY-MM-DD')} is after ${d3.format('YYYY-MM-DD')}`);
}

In the example, we compare three dates. 

$ node queries.js
these are same dates
2018-05-19 is before 2018-05-20
2018-05-20 is before 2018-05-22

This is the output.

The isBetween() function checks if a date is in the given date range.

between.js
const dayjs = require('dayjs');

const isBetween = require('dayjs/plugin/isBetween');

dayjs.extend(isBetween);

let d1 = dayjs("2018-05-19");

if (d1.isBetween('2018-05-10', '2018-05-25')) {

    console.log("The day is within the date range");
} else {

    console.log("The day is not within the date range");
}

The example uses the isBetween() function to determine if a date is within the specified date range. For this example we need the isBetween plugin.

Day.js unix time

Unix time is the number of seconds since the Unix epoch. The unix() function returns the value of time in seconds since 0 hours, 0 minutes, 0 seconds, January 1, 1970, Coordinated Universal Time.

unixtime.js
const dayjs = require('dayjs');

let unixTime_s = dayjs().unix();
console.log(unixTime_s);

let unixTime_ms = dayjs().valueOf();
console.log(unixTime_ms);

let unixTime2 = dayjs(1);
console.log(unixTime2.format('YYYY-DD-MM'));

In the example, we get the current unix time and convert unix time 1 s to human readable format. 

let unixTime_s = dayjs().unix();

We get the Unix time with unix() function. The returned value is the number of seconds elapsed from the start of the Unix Epoch. 

let unixTime_ms = dayjs().valueOf();

With the valueOf() function, we get the Unix time in milliseconds. 

let unixTime2 = dayjs(1);
console.log(unixTime2.format('YYYY-DD-MM'));

We get the unix time of 1 s and output it in the given format. 

$ node unix_time.js
1557825803
1557825803834
1970-01-01

This is a sample output.

Day.js leap year

A leap year is a year containing an additional day. The reason for an extra day in the calendar is the difference between the astronomical and the calendar year. We need to add the isLeapYear plugin.

leap_year.js
const dayjs = require('dayjs');
const isLeapYear = require('dayjs/plugin/isLeapYear');

dayjs.extend(isLeapYear)

// Assume year >= 1582 in the Gregorian calendar.

let years = [ 2000, 2002, 2004, 2008, 2012, 2016, 2020,
    1900, 1800, 1600 ];

for (year of years) {

    let ym = dayjs([year]);

    if (ym.isLeapYear()) {

        console.log(`${year} is a leap year`);
    } else {
        
        console.log(`${year} is not a leap year`);
    }
}

In the example, we have an array of years. We determine which years are leap years. 

if (ym.isLeapYear()) {

We determine if a year is a leap year with the isLeapYear() function. 

$ node leap_year.js
2000 is a leap year
2002 is not a leap year
2004 is a leap year
2008 is a leap year
2012 is a leap year
2016 is a leap year
2020 is a leap year
1900 is not a leap year
1800 is not a leap year
1600 is a leap year

This is the output.

In this tutorial, we have worked with date and time in JavaScript using the Day.js library.

List all JavaScript tutorials.