How to create an accurate timer in javascript?

I need to create a simple but accurate timer.

This is my code:

var seconds = 0;
setInterval(function() {
timer.innerHTML = seconds++;
}, 1000);

After exactly 3600 seconds, it prints about 3500 seconds.

• Why is it not accurate?

• How can I create an accurate timer?

> Why is it not accurate?

Because you are using setTimeout() or setInterval(). They cannot be trusted, there are no accuracy guarantees for them. They are allowed to lag arbitrarily, and they do not keep a constant pace but tend to drift (as you have observed).

> How can I create an accurate timer?

Use the Date object instead to get the (millisecond-)accurate, current time. Then base your logic on the current time value, instead of counting how often your callback has been executed.

For a simple timer or clock, keep track of the time difference explicitly:

var start = Date.now();
setInterval(function() {
    var delta = Date.now() - start; // milliseconds elapsed since start
    …
    output(Math.floor(delta / 1000)); // in seconds
    // alternatively just show wall clock time:
    output(new Date().toUTCString());
}, 1000); // update about every second

Now, that has the problem of possibly jumping values. When the interval lags a bit and executes your callback after 990, 1993, 2996, 3999, 5002 milliseconds, you will see the second count 0, 1, 2, 3, 5 (!). So it would be advisable to update more often, like about every 100ms, to avoid such jumps.

However, sometimes you really need a steady interval executing your callbacks without drifting. This requires a bit more advanced strategy (and code), though it pays out well (and registers less timeouts). Those are known as self-adjusting timers. Here the exact delay for each of the repeated timeouts is adapted to the actually elapsed time, compared to the expected intervals:

var interval = 1000; // ms
var expected = Date.now() + interval;
setTimeout(step, interval);
function step() {
    var dt = Date.now() - expected; // the drift (positive for overshooting)
    if (dt > interval) {
        // something really bad happened. Maybe the browser (tab) was inactive?
        // possibly special handling to avoid futile "catch up" run
    }
    … // do what is to be done

    expected += interval;
    setTimeout(step, Math.max(0, interval - dt)); // take into account drift
}

I'ma just build on Bergi's answer (specifically the second part) a little bit because I really liked the way it was done, but I want the option to stop the timer once it starts (like clearInterval() almost). Sooo... I've wrapped it up into a constructor function so we can do 'objecty' things with it.

1. Constructor

Alright, so you copy/paste that...

/**
 * Self-adjusting interval to account for drifting
 * 
 * @param {function} workFunc  Callback containing the work to be done
 *                             for each interval
 * @param {int}      interval  Interval speed (in milliseconds)
 * @param {function} errorFunc (Optional) Callback to run if the drift
 *                             exceeds interval
 */
function AdjustingInterval(workFunc, interval, errorFunc) {
	var that = this;
	var expected, timeout;
	this.interval = interval;

	this.start = function() {
		expected = Date.now() + this.interval;
		timeout = setTimeout(step, this.interval);
	}

	this.stop = function() {
		clearTimeout(timeout);
	}

	function step() {
		var drift = Date.now() - expected;
		if (drift > that.interval) {
			// You could have some default stuff here too...
			if (errorFunc) errorFunc();
		}
		workFunc();
		expected += that.interval;
		timeout = setTimeout(step, Math.max(0, that.interval-drift));
	}
}

2. Instantiate

Tell it what to do and all that...

// For testing purposes, we'll just increment
// this and send it out to the console.
var justSomeNumber = 0;

// Define the work to be done
var doWork = function() {
	console.log(++justSomeNumber);
};

// Define what to do if something goes wrong
var doError = function() {
	console.warn('The drift exceeded the interval.');
};

// (The third argument is optional)
var ticker = new AdjustingInterval(doWork, 1000, doError);

3. Then do... stuff

// You can start or stop your timer at will
ticker.start();
ticker.stop();

// You can also change the interval while it's in progress
ticker.interval = 99;

I mean, it works for me anyway. If there's a better way, lemme know.

Bergi's answer pinpoints exactly why the timer from the question is not accurate. Here's my take on a simple JS timer with start, stop, reset and getTime methods:

class Timer {
  constructor () {
    this.isRunning = false;
    this.startTime = 0;
    this.overallTime = 0;
  }

  _getTimeElapsedSinceLastStart () {
    if (!this.startTime) {
      return 0;
    }
  
    return Date.now() - this.startTime;
  }

  start () {
    if (this.isRunning) {
      return console.error('Timer is already running');
    }

    this.isRunning = true;

    this.startTime = Date.now();
  }

  stop () {
    if (!this.isRunning) {
      return console.error('Timer is already stopped');
    }

    this.isRunning = false;

    this.overallTime = this.overallTime + this._getTimeElapsedSinceLastStart();
  }

  reset () {
    this.overallTime = 0;

    if (this.isRunning) {
      this.startTime = Date.now();
      return;
    }

    this.startTime = 0;
  }

  getTime () {
    if (!this.startTime) {
      return 0;
    }

    if (this.isRunning) {
      return this.overallTime + this._getTimeElapsedSinceLastStart();
    }

    return this.overallTime;
  }
}

const timer = new Timer();
timer.start();
setInterval(() => {
  const timeInSeconds = Math.round(timer.getTime() / 1000);
  document.getElementById('time').innerText = timeInSeconds;
}, 100)

<p>Elapsed time: <span id="time">0</span>s</p>

The snippet also includes a solution for your problem. So instead of incrementing seconds variable every 1000ms interval, we just start the timer and then every 100ms* we just read elapsed time from the timer and update the view accordingly.

* - makes it more accurate than 1000ms

To make your timer more accurate, you would have to round

Most of the timers in the answers here will linger behind the expected time because they set the "expected" value to the ideal and only account for the delay that the browser introduced before that point. This is fine if you just need accurate intervals, but if you are timing relative to other events then you will (nearly) always have this delay.

To correct it, you can keep track of the drift history and use it to predict future drift. By adding a secondary adjustment with this preemptive correction, the variance in the drift centers around the target time. For example, if you're always getting a drift of 20 to 40ms, this adjustment would shift it to -10 to +10ms around the target time.

Building on Bergi's answer, I've used a rolling median for my prediction algorithm. Taking just 10 samples with this method makes a reasonable difference.

var interval = 200; // ms
var expected = Date.now() + interval;

var drift_history = [];
var drift_history_samples = 10;
var drift_correction = 0;

function calc_drift(arr){
  // Calculate drift correction.

  /*
  In this example I've used a simple median.
  You can use other methods, but it's important not to use an average. 
  If the user switches tabs and back, an average would put far too much
  weight on the outlier.
  */

  var values = arr.concat(); // copy array so it isn't mutated
  
  values.sort(function(a,b){
    return a-b;
  });
  if(values.length ===0) return 0;
  var half = Math.floor(values.length / 2);
  if (values.length % 2) return values[half];
  var median = (values[half - 1] + values[half]) / 2.0;
  
  return median;
}

setTimeout(step, interval);
function step() {
  var dt = Date.now() - expected; // the drift (positive for overshooting)
  if (dt > interval) {
    // something really bad happened. Maybe the browser (tab) was inactive?
    // possibly special handling to avoid futile "catch up" run
  }
  // do what is to be done
       
  // don't update the history for exceptionally large values
  if (dt <= interval) {
    // sample drift amount to history after removing current correction
    // (add to remove because the correction is applied by subtraction)
      drift_history.push(dt + drift_correction);

    // predict new drift correction
    drift_correction = calc_drift(drift_history);

    // cap and refresh samples
    if (drift_history.length >= drift_history_samples) {
      drift_history.shift();
    }    
  }
   
  expected += interval;
  // take into account drift with prediction
  setTimeout(step, Math.max(0, interval - dt - drift_correction));
}

I agree with Bergi on using Date, but his solution was a bit of overkill for my use. I simply wanted my animated clock (digital and analog SVGs) to update on the second and not overrun or under run creating obvious jumps in the clock updates. Here is the snippet of code I put in my clock update functions:

    var milliseconds = now.getMilliseconds();
    var newTimeout = 1000 - milliseconds;
    this.timeoutVariable = setTimeout((function(thisObj) { return function() { thisObj.update(); } })(this), newTimeout);

It simply calculates the delta time to the next even second, and sets the timeout to that delta. This syncs all of my clock objects to the second. Hope this is helpful.

Here's a solution that pauses when the window is hidden, and can be cancelled with an abort controller.

function animationInterval(ms, signal, callback) {
  const start = document.timeline.currentTime;

  function frame(time) {
    if (signal.aborted) return;
    callback(time);
    scheduleFrame(time);
  }

  function scheduleFrame(time) {
    const elapsed = time - start;
    const roundedElapsed = Math.round(elapsed / ms) * ms;
    const targetNext = start + roundedElapsed + ms;
    const delay = targetNext - performance.now();
    setTimeout(() => requestAnimationFrame(frame), delay);
  }

  scheduleFrame(start);
}

Usage:

const controller = new AbortController();

// Create an animation callback every second:
animationInterval(1000, controller.signal, time => {
  console.log('tick!', time);
});

// And stop it sometime later:
controller.abort();

This is an old question but figured I'd share some code I use sometimes:

function Timer(func, delay, repeat, runAtStart)
{
    this.func = func;
    this.delay = delay;
    this.repeat = repeat || 0;
    this.runAtStart = runAtStart;

    this.count = 0;
    this.startTime = performance.now();

    if (this.runAtStart)
        this.tick();
    else
    {
        var _this = this;
        this.timeout = window.setTimeout( function(){ _this.tick(); }, this.delay);
    }
}
Timer.prototype.tick = function()
{
    this.func();
    this.count++;

    if (this.repeat === -1 || (this.repeat > 0 && this.count < this.repeat) )
    {
        var adjustedDelay = Math.max( 1, this.startTime + ( (this.count+(this.runAtStart ? 2 : 1)) * this.delay ) - performance.now() );
        var _this = this;
        this.timeout = window.setTimeout( function(){ _this.tick(); }, adjustedDelay);
    }
}
Timer.prototype.stop = function()
{
    window.clearTimeout(this.timeout);
}

Example:

time = 0;
this.gameTimer = new Timer( function() { time++; }, 1000, -1);

Self-corrects the setTimeout, can run it X number of times (-1 for infinite), can start running instantaneously, and has a counter if you ever need to see how many times the func() has been run. Comes in handy.

Edit: Note, this doesn't do any input checking (like if delay and repeat are the correct type. And you'd probably want to add some kind of get/set function if you wanted to get the count or change the repeat value.

Inspired by Bergi's answer I created the following complete non drifting timer. What I wanted was a way to set a timer, stop it, and do this simply.

var perfectTimer = {                                                              // Set of functions designed to create nearly perfect timers that do not drift
	timers: {},                                                                     // An object of timers by ID
  nextID: 0,                                                                      // Next available timer reference ID
  set: (callback, interval) => {                                                  // Set a timer
    var expected = Date.now() + interval;                                         // Expected currect time when timeout fires
    var ID = perfectTimer.nextID++;                                               // Create reference to timer
    function step() {                                                             // Adjusts the timeout to account for any drift since last timeout
      callback();                                                                 // Call the callback
      var dt = Date.now() - expected;                                             // The drift (ms) (positive for overshooting) comparing the expected time to the current time
      expected += interval;                                                       // Set the next expected currect time when timeout fires
      perfectTimer.timers[ID] = setTimeout(step, Math.max(0, interval - dt));     // Take into account drift
    }
    perfectTimer.timers[ID] = setTimeout(step, interval);                         // Return reference to timer
    return ID;
  },
  clear: (ID) => {                                                                // Clear & delete a timer by ID reference
    if (perfectTimer.timers[ID] != undefined) {                                   // Preventing errors when trying to clear a timer that no longer exists
      console.log('clear timer:', ID);
      console.log('timers before:', perfectTimer.timers);
      clearTimeout(perfectTimer.timers[ID]);                                      // Clear timer
      delete perfectTimer.timers[ID];                                             // Delete timer reference
      console.log('timers after:', perfectTimer.timers);
    }
	}		
}




// Below are some tests
var timerOne = perfectTimer.set(() => {
	console.log(new Date().toString(), Date.now(), 'timerOne', timerOne);
}, 1000);
console.log(timerOne);
setTimeout(() => {
	perfectTimer.clear(timerOne);
}, 5000)

var timerTwo = perfectTimer.set(() => {
	console.log(new Date().toString(), Date.now(), 'timerTwo', timerTwo);
}, 1000);
console.log(timerTwo);

setTimeout(() => {
	perfectTimer.clear(timerTwo);
}, 8000)

Modern, Fully Programmable Timer

This timer takes a frequency in Hertz, and a callback that can take up to four arguments, the current frame index, the current time, the time that the current frame would have ideally occurred at, and a reference to the timer instance (so the caller and callback can both access its methods).

Note: All times are based on performance.now, and are relative to the moment that the page loaded.

Timer instances have three API methods:

• stop: Takes no args. Kills the timer immediately (and permanently). Returns the frame index for the next frame (the cancelled frame).
• adapt: Takes a frequency in Hertz and adapts the timer to it, beginning from the next frame. Returns the implied interval in milliseconds.
• redefine: Takes a new callback function. Swaps it with the current callback. Effects the next frame. Returns undefined.

Note: The tick method passes this around explicitly (as self) to work around the problem of this referencing window when the tick method is invoked via setTimeout.

class ProgrammableTimer {

    constructor(hertz, callback) {

        this.target = performance.now();     // target time for the next frame
        this.interval = 1 / hertz * 1000;    // the milliseconds between ticks
        this.callback = callback;
        this.stopped = false;
        this.frame = 0;

        this.tick(this);
    }

    tick(self) {

        if (self.stopped) return;

        const currentTime = performance.now();
        const currentTarget = self.target;
        const currentInterval = (self.target += self.interval) - currentTime;

        setTimeout(self.tick, currentInterval, self);
        self.callback(self.frame++, currentTime, currentTarget, self);
    }

    stop() { this.stopped = true; return this.frame }

    adapt(hertz) { return this.interval = 1 / hertz * 1000 }

    redefine(replacement) { this.callback = replacement }
}

Doesn't get much more accurate than this.

var seconds = new Date().getTime(), last = seconds,

intrvl = setInterval(function() {
    var now = new Date().getTime();

    if(now - last > 5){
        if(confirm("Delay registered, terminate?")){
            clearInterval(intrvl);
            return;
        }
    }

    last = now;
    timer.innerHTML = now - seconds;

}, 333);

As to why it is not accurate, I would guess that the machine is busy doing other things, slowing down a little on each iteration adds up, as you see.

One of my simplest implementations is down below. It can even survive page reloads. :-

Code pen: https://codepen.io/shivabhusal/pen/abvmgaV

$(function() {
  var TTimer = {
    startedTime: new Date(),
    restoredFromSession: false,
    started: false,
    minutes: 0,
    seconds: 0,
    
    tick: function tick() {
      // Since setInterval is not reliable in inactive windows/tabs we are using date diff.
      var diffInSeconds = Math.floor((new Date() - this.startedTime) / 1000);
      this.minutes = Math.floor(diffInSeconds / 60);
      this.seconds = diffInSeconds - this.minutes * 60;
      this.render();
      this.updateSession();
    },
    
    utilities: {
      pad: function pad(number) {
        return number < 10 ? '0' + number : number;
      }
    },
    
    container: function container() {
      return $(document);
    },
    
    render: function render() {
      this.container().find('#timer-minutes').text(this.utilities.pad(this.minutes));
      this.container().find('#timer-seconds').text(this.utilities.pad(this.seconds));

    },
    
    updateSession: function updateSession() {
      sessionStorage.setItem('timerStartedTime', this.startedTime);
    },
    
    clearSession: function clearSession() {
      sessionStorage.removeItem('timerStartedTime');
    },
    
    restoreFromSession: function restoreFromSession() {
      // Using sessionsStorage to make the timer persistent
      if (typeof Storage == "undefined") {
        console.log('No sessionStorage Support');
        return;
      }

      if (sessionStorage.getItem('timerStartedTime') !== null) {
        this.restoredFromSession = true;
        this.startedTime = new Date(sessionStorage.getItem('timerStartedTime'));
      }
    },
    
    start: function start() {
      this.restoreFromSession();
      this.stop();
      this.started = true;
      this.tick();
      this.timerId = setInterval(this.tick.bind(this), 1000);
    },
    
    stop: function stop() {
      this.started = false;
      clearInterval(this.timerId);
      this.render();
    }
  };

  TTimer.start();

});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>

<h1>
  <span id="timer-minutes">00</span> :
  <span id="timer-seconds">00</span>

</h1>

driftless is a drop-in replacement for setInterval that mitigates drift. Makes life easy, import the npm package, then use it like setInterval / setTimeout:

setDriftlessInterval(() => {
    this.counter--;
}, 1000);

setDriftlessInterval(() => {
    this.refreshBounds();
}, 20000);

you can use a function called setTimeout that we can use to set the countdown.

Firstly, create a javascript snippet and add it to your page as follows;

var remainingTime = 30;
    var elem = document.getElementById('countdown_div');
    var timer = setInterval(countdown, 1000); //set the countdown to every second
    function countdown() {
      if (remainingTime == -1) {
        clearTimeout(timer);
        doSomething();
      } else {
        elem.innerHTML = remainingTime + ' left';
        remainingTime--; //we subtract the second each iteration
      }
    }

Source + more details -> https://www.growthsnippets.com/30-second-countdown-timer-javascript/

Many of these answers here are great, but they typically their code examples are pages and pages of code (the good ones even have instructions on the best way to copy/paste it all). I just wanted to understand this problem with a very simple example.

Working Demo

var lastpause = 0;
var totaltime = 0;

function goFunction(e) {
    if(this.innerText == 'Off - Timer Not Going') {
        this.innerText = 'On - Timer Going';
    } else {
        totaltime += Date.now() - lastpause;
        this.innerText = 'Off - Timer Not Going';
    }
    lastpause = Date.now();
    document.getElementById('count').innerText = totaltime + ' milliseconds.';
}

document.getElementById('button').addEventListener('click', goFunction);

<button id="button">Off - Timer Not Going</button> <br>
Seconds: <span id="count">0 milliseconds.</span>

Explanation of Demo

• totaltime — This is the total time calculated.
• lastpause — This is the only real temporary variable we have. Whenever someone hits pause, we set lastpause to Date.now(). When someone unpauses, and re-pauses again, we calculate the time diff of Date.now() subtracted from the last pause.

We only need those two variables: Our total and the last time we stopped the timer. The other answers seem to use this approach, but I wanted a compact explanation.