Understand JavaScript Date Object with Examples (for JavaScript Interviews)

Working with date in javascript has very common use-case in web applications, and date-related operations can be achieved using Javascript Date Object. There are many methods supported by Date Object which makes it very easy to work with date in javascript.

Table of Content

1. Ways to Create JavaScript Date Object
   • No parameter new Date()
   • Epoch value parameter - new Date(epoch value)
   • Date String - new Date(date string)
   • Date time individual value - new Date(year, month, <day>, <hour>, <minute>, <second>, <millisecond>)
2. Commonly used Static & Instance Methods of Date Object
   • Static Date.now()
   • Static Date.parse()
   • getDate() and getTime()
   • getDay(), getMonth() & getFullYear()
   • getHours(), getMinutes(), getSeconds() & getMilliseconds()
3. Common use-cases of javascript date object in Frontend Interviews
   • Find the time difference between 2 time stamps in javascript

Ways to Create JavaScript Date Object

Date object supports 4 types of constructor functions to create a new object 

💡 Number of milliseconds since midnight at the beginning of January 1, 1970, UTC — also known as the epoch

1. No parameter new Date()

This constructor doesn't take any parameter and provides a string representation of date time.

Example

const date = new Date();
// "2025-01-05T11:29:53.357Z"

2. Epoch value parameter - new Date(epoch value)

This constructor takes the integer epoch value (in milliseconds), this input is the same which returned by Date.now() method. It returns the date-time in string format.

Example

// Epoc value - 1736076951966 in millisecond

const date = new Date(1736076951966);
// 2025-01-05T11:35:51.966Z

3. Date string - new Date(date string)

This constructor takes the date string and returns the date object representing the same date string.

Example

const date = new Date("2025-01-07T11:45:51.966Z");

// 2025-01-07T11:45:51.966Z

4. Date time Individual Value - new Date(year, month, <day>, <hour>, <minute>, <second>, <millisecond>)

This parameter can take the individual values of date time parameters. In this date, constructor year and month parameters are mandatory to pass and other parameters are optional.

Example

const date = new Date(2025, 01, 04, 3, 24, 0);

// 2025-02-03T21:54:00.000Z

Commonly Used Static & Instance Methods of Date Object in JavaScript

There are many Static and instance methods supported by date objects, lets check out the most common static and instance methods of Date Objects in javascript. These methods will help to handle any date-related use-case in Machine Coding and javascript interview rounds.  

1. Static Date.now()

static method now() returns the epoch time in milliseconds, this method is used generally to find the time difference in milliseconds between the 2 timestamps  

const date = Date.now() 

console.log('Epoc time in milliseconds: ', date);
// Epoc time in milliseconds: 1736080469441

2. Static Date.parse()

parse() static method parses the input time and provides the corresponding epoch time, this is the same as using getTime() method to get epoch time for a given input date.

Date.parse("2025-01-07T11:45:51.966Z");
// 1736250351966

const date = new Date("2025-01-07T11:45:51.966Z");
date.getTime() 
// 1736250351966

3. getDate() & getTime()

const date = new Date("2025-01-07T11:45:51.966Z");

date.getDate() // 7
date.getTime() // 1736250351966

4. getDay(), getMonth() & getFullYear()

const date = new Date("2025-01-07T11:45:51.966Z");

date.getDay() // 2
date.getMonth() // 0
date.getFullYear() // 2025

5. getHours(), getMinutes(), getSeconds() & getMilliseconds()

const date = new Date("2025-01-07T11:45:51.966Z");

date.getHours(); // 17
date.getMinutes(); // 15
date.getSeconds(); // 51
date.getMilliseconds() // 966

Common use-cases of JavaScript Date Object in Frontend Interview

1. Find the time difference between 2 timestamps in javascript

static method Date.now() can be used to find the epoch time in milliseconds, and can be used to find the difference between 2 timestamps

Example

const epochTime1 = Date.now();
console.log('epochTime1 = ',epochTime1);

let sum = 0;
for(let i=0; i<1000000; i++){
  // do nothing
  sum = sum + i;
}

const epochTime2 = Date.now();
console.log('epochTime2 = ',epochTime2);

console.log('Time difference - ', epochTime2 - epochTime1);

// Output
epochTime1 = 1736081427953
epochTime2 = 1736081427962
Time difference -  9