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A javscript timer/countdown clock.

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Based on an idea by James Edwards:

Readme Contents

What's so good about it?

  • Pure Javascript - no dependencies
  • Self-correcting time based on the system clock - won't go out of time unlike clocks based solely on setInterval or setTimeout (see the link above).
  • It can be used to count up from any arbitrary time (or 0:00) or countdown from a given time.
  • It can call a callback function every tick (10 milliseconds) and (for countdown clocks) when the clock reaches 0:00.
  • It's about as accurate a clock as you can get with Javascript.

When would I use it?

  • Countdown counters, e.g. "site will launch in..."
  • Timers
  • Accurate timing of any repeated action

How do I get it?

You probably use a dependency manager, such as NPM:

npm install tocktimer

or Yarn

yarn add tocktimer

How do I use it?

Tock.js works behind the scenes - it doesn't alter anything on screen - so here I'll show how to make a stop-watch that updates in real-time on screen.

1) Make some html to show the clock.

<button id="start">Start</button>
<button id="stop">Stop</button>
<input id="clock" value="10:00">

2) Instantiate a Tock

First create a new instance of Tock and assign it to a variable called timer.

const timer = new Tock();

This will give you a clock that will count up from 00:00 when the start() method is called. The stop() and reset() methods can also be used.

For more control we can pass in some options. Note that all options are...optional.

const timer = new Tock({
  countdown: true,
  interval: 10,
  callback: someCallbackFunction,
  complete: someCompleteFunction

2) Add some controls

You'll need some way of controlling your clock. Let's set add a start button. Note that we get the time from the clock input and pass it to the start function as the start time.

document.getElementById('start').addEventListener('click', () => {

Now add a stop button

document.getElementById('stop').addEventListener('click', () => {

If you're not using a countdown clock you can make a reset button, too.

document.getElementById('reset').addEventListener('click', () => {

You could also create a reset button if you are using a countdown clock, but that's beyond the scope of this walkthrough. The tools are there. Do with them what you can. After this next section you're on your own. Good luck. We're all counting on you.


  • countdown boolean Default: false. If true, the clock will count down from a given time, otherwise it will count up from 0:00.
  • interval integer Default: 10. How often, in milliseconds, that the clock will tick.
  • callback function Default: null (see below)
  • complete function Default: null (see below)


The callback option is a function that will be called once every interval milliseconds.

Here we'll use the lap() method to get the current clock time (in milliseconds). We'll then pass that through msToTime() to format it nicely before displaying it in the input field.

callback: () => {
  var current_time = timer.msToTime(timer.lap());
  document.getElementById('clock').value = current_time;

If countdown is true you can also pass in a function to call once the countdown reaches zero.

complete: () => {
  alert("Time's up!");


  • start(time) - Start the timer
    • time (optional) can be either a countdown value or a starting value.

      If a countdown is true then set time to count down from.

      If a countdown is false or not time will control start time to count up from.

      Both timer types allow time as an integer number of milliseconds or as as string - see timeToMS below.

  • stop() - Stop the clock and clear the timeout
  • pause() - Stop the clock if it's running, continue clock if paused
  • reset() - Reset times to zero
    • Note: Countdown clocks need a duration to be passed to start() after reset() is called.
  • lap() - Return elapsed time in milliseconds


Note: Tock is designed to work with millisecond values. These conversion methods are provided as basic helpers and may be removed entirely in later versions. If you want more complex or custom formatting, you might want to use date-fns.

  • msToTime(ms) - Convert number of milliseconds to a MM:SS time string
    • Won't handle times greater than 1 hour
  • msToTimecode(ms, show_ms) - Convert number of milliseconds to timecode string
    • ms - number of milliseconds
    • show_ms - Optional. If true, return an HH:MM:SS:mmm format otherwise HH:MM:SS
  • timeToMS(time) - Convert a time string to a number of milliseconds
    • string - should be a duration as a string of form:
      • MM:SS
      • MM:SS:ms
      • HH:MM:SS
    • Alternatively a time in the future can be provided using the form yyyy-mm-dd The difference between this time and present will be returned.
    • If the input cannot be recognized as one of the above then 0 is returned


I'm using Grunt for task running and Mocha for testing.

Get all dependencies with:

npm install

Run all tasks:


Run all tasks automatically when you save.

grunt watch


MIT License.



Last updated on 28 Jun 2022

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