Vanilla JavaScript countdown clock


Creating a cool JavaScript countdown timer

14 Nov, 2020 · 4 min read

A time ago, we made this cool year loading bar, and this made me think a countdown timer would also be cool.

Since I’m getting married next year, let’s use that as an example.

We will be creating an end date, and every second we will check how long away this is.

What you’ll learn in this article:

  • ✅ Working with JavaScript dates
  • ✅ Calculating date differences
  • ✅ Converting milliseconds to readable formats
  • ✅ Styling our clock

The result will look like this:

See the Pen Vanilla JavaScript countdown clock by Chris Bongers (@rebelchris) on CodePen.

HTML Structure

Let’s start by defining our HTML structure.

  <h1>The big day</h1>
  <p>Nicole & Chris wedding</p>
  <p id="done"></p>
    <li><span id="days">0</span> Days</li>
    <li><span id="hours">0</span> Hours</li>
    <li><span id="minutes">0</span> Minutes</li>
    <li><span id="seconds">0</span> Seconds</li>

We will have a title, an intro paragraph, and an empty done div. This done div will be used if the timer expires.

Then we have a list with days, hours, minutes, and seconds. Default on 0 in case the date expires.

Adding some CSS for styling

Now, of course, we want this to look a bit nicer.

body {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  font-family: Roboto, 'Helvetica Neue', Arial, sans-serif;
  text-align: center;
  margin: 0;
  padding: 0;
  background: #8fbfe0;
h1 {
  font-size: 3rem;
  margin-top: 0;
ul {
  display: flex;
  list-style: none;
  margin: 0;
  padding: 0;
  margin-top: 2rem;
ul li {
  background: #7c77b9;
  border-radius: 10px;
  padding: 1rem;
  display: flex;
  flex-direction: column;
  margin: 0 0.5rem;
  color: #8fbfe0;
  box-shadow: 0px 0px 5px rgba(0, 0, 0, 0.5);
ul li span {
  font-size: 2rem;

You’ll see that we remove the main styling from the ul to use Flexbox to center it, and space the elements. Then we add a box shadow and some colors to make it pop more.

JavaScript countdown timer

Now the JavaScript part.

Let’s first define our end date:

const end = new Date('May 03, 2021 00:00:00').getTime();

We define the date as a Date object and use the getTime function to get the milliseconds. We do this because it’s easier to count with.

Now let’s get all our output span elements.

const dayEl = document.getElementById('days');
const hoursEl = document.getElementById('hours');
const minutesEl = document.getElementById('minutes');
const secondsEl = document.getElementById('seconds');

The last set of variables we can define to make it easier for ourselves are the time fragment pieces.

const seconds = 1000;
const minutes = seconds * 60;
const hours = minutes * 60;
const days = hours * 24;

What this means:

  • 1000 seconds = 1 millisecond, so if we do (time / 1000) we get our seconds
  • (seconds * 60) = 1 minute (60.000 milliseconds)
  • (minutes * 60) = 1 hour because 1 hour has 60 seconds.
  • (hours * 24) = 1 day because 1 day has 24 hours

We must create a setInterval function to run every second.

const x = setInterval(function () {
  // Code here
}, seconds);

We attach it to a const to be able to unset it if it’s no longer needed.

Let’s get the current timestamp and the difference between our end date and now.

let now = new Date().getTime();
const difference = end - now;

The difference will now have the difference in milliseconds and our set date.

Let’s first check if it’s not already expired.

if (difference < 0) {
  document.getElementById('done').innerHTML = "We're married! 🎉";

Here, we check if the difference is smaller than 0, then clear our interval so it won’t run again. We also use a return to stop the rest of the function from running.

Now all that’s left is to show the correct numbers for each element.

dayEl.innerText = Math.floor(difference / days);
hoursEl.innerText = Math.floor((difference % days) / hours);
minutesEl.innerText = Math.floor((difference % hours) / minutes);
secondsEl.innerText = Math.floor((difference % minutes) / seconds);

As mentioned in our example, we return a floored value of the difference converted to each respectable element.

Note that the % is used to get the remaining back.

For example, let’s say our difference in milliseconds is 15091810828.

  • Days: (15091810828 / 86400000) = 174
  • Hours: (15091810828 % 86400000) = 58210828 => (58210828 / 3600000) = 16
  • Minutes: (15091810828 % 3600000) = 610828 => (610828 / 60000) = 10
  • Seconds: (15091810828 % 60000) = 10828 => (10828 / 1000) = 10

This might be a bit much. I’d advise you to write down one number and calculate it on paper to get a good feel for what’s happening.

There we go. We now have a countdown timer in JavaScript. Let me know what you create with this cool piece of code.

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