This article explains how to build countdown timer in Javascript. It will make your code faster, website will perform better. You can control the way you want when you build the clock.

Here are the steps to creating a basic clock.

• Set a valid end date.
• Calculate the time remaining.
• Convert the time to a usable format.
• Output the clock data as a reusable object.
• Display the clock on the page, and stop the clock when it reaches zero.

Set a Valid End Date

First, you’ll need to set a valid end date. This should be a string in any of the formats understood by JavaScript’s Date.parse() method. For example:
const deadline = '2015-12-31';

Calculate the Time Remaining

The next step is to calculate the time remaining. We need to write a function that takes a string representing a given end time (as outlined above). We then calculate the difference between that time and the current time. Here’s what that looks like:

function getTimeRemaining(endtime){
  const total = Date.parse(endtime) - Date.parse(new Date());
  const seconds = Math.floor( (total/1000) % 60 );
  const minutes = Math.floor( (total/1000/60) % 60 );
  const hours = Math.floor( (total/(1000*60*60)) % 24 );
  const days = Math.floor( total/(1000*60*60*24) );

  return {
    total,
    days,
    hours,
    minutes,
    seconds
  };
}

First, we’re creating a variable total, to hold the remaining time until the deadline. The Date.parse() function converts a time string into a value in milliseconds. This allows us to subtract two times from each other and get the amount of time in between.

Convert the Time to a Usable Format

Now we want to convert the milliseconds to days, hours, minutes, and seconds. Let’s use seconds as an example:

Let’s break down what’s going on here.

Divide milliseconds by 1000 to convert to seconds: (t/1000)
Divide the total seconds by 60 and grab the remainder. You don’t want all of the seconds, just those remaining after the minutes have been counted: (t/1000) % 60
Round this down to the nearest whole number. This is because you want complete seconds, not fractions of seconds: Math.floor( (t/1000) % 60 )
Repeat this logic to convert the milliseconds to minutes, hours, and days.

Output the Clock Data as a Reusable Object

With the days, hours, minutes, and seconds prepared, we’re now ready to return the data as a reusable object:
This object allows you to call your function and get any of the calculated values. Here’s an example of how you’d get the remaining minutes:
getTimeRemaining(deadline).seconds

Display the Clock and Stop It When It Reaches Zero

Now that we have a function that spits out the days, hours, minutes, and seconds remaining, we can build our clock. First we’ll create the following HTML element to hold our clock:

<div id="clockdiv"></div>
Then we’ll write a function that outputs the clock data inside our new div:

function initializeClock(id, endtime) {
  const clock = document.getElementById(id);
  const timeinterval = setInterval(() => {
    const t = getTimeRemaining(endtime);
    clock.innerHTML = 'days: ' + t.days + '<br>' +
                      'hours: '+ t.hours + '<br>' +
                      'minutes: ' + t.minutes + '<br>' +
                      'seconds: ' + t.seconds;
    if (t.total <= 0) {
      clearInterval(timeinterval);
    }
  },1000);
}

This function takes two parameters. These are the id of the element that contains our clock, and the countdown’s end time. Inside the function, we’ll declare a clock variable and use it to store a reference to our clock container div. This means we don’t have to keep querying the DOM.

Next, we’ll use setInterval to execute an anonymous function every second. This function will do the following:

• Calculate the remaining time.
• Output the remaining time to our div.
• If the remaining time gets to zero, stop the clock.

At this point, the only remaining step is to run the clock like so:
initializeClock('clockdiv', deadline);

References:
https://www.sitepoint.com/build-javascript-countdown-timer-no-dependencies/
https://codepen.io/SitePoint/pen/NWxKgxN