Develop Reference

Javascript Date.now() function [duplicate]

I got this code over here:
var date = new Date();
setTimeout(function(e) {
var currentDate = new Date();
if(currentDate - date >= 1000) {
console.log(currentDate, date);
console.log(currentDate-date);
}
else {
console.log("It was less than a second!");
console.log(currentDate-date);
}
}, 1000);
In my computer, it always executes correctly, with 1000 in the console output. Interestedly in other computer, the same code, the timeout callback starts in less than a second and the difference of currentDate - date is between 980 and 998.
I know the existence of libraries that solve this inaccuracy (for example, Tock).
Basically, my question is: What are the reasons because setTimeout does not fire in the given delay? Could it be the computer that is too slow and the browser automatically tries to adapt to the slowness and fires the event before?
PS: Here is a screenshot of the code and the results executed in the Chrome JavaScript console:

It's not supposed to be particularly accurate. There are a number of factors limiting how soon the browser can execute the code; quoting from MDN:
In addition to "clamping", the timeout can also fire later when the page (or the OS/browser itself) is busy with other tasks.
In other words, the way that setTimeout is usually implemented, it is just meant to execute after a given delay, and once the browser's thread is free to execute it.
However, different browsers may implement it in different ways. Here are some tests I did:
var date = new Date();
setTimeout(function(e) {
var currentDate = new Date();
console.log(currentDate-date);
}, 1000);
// Browser Test1 Test2 Test3 Test4
// Chrome 998 1014 998 998
// Firefox 1000 1001 1047 1000
// IE 11 1006 1013 1007 1005
Perhaps the < 1000 times from Chrome could be attributed to inaccuracy in the Date type, or perhaps it could be that Chrome uses a different strategy for deciding when to execute the code—maybe it's trying to fit it into the a nearest time slot, even if the timeout delay hasn't completed yet.
In short, you shouldn't use setTimeout if you expect reliable, consistent, millisecond-scale timing.

In general, computer programs are highly unreliable when trying to execute things with higher precision than 50 ms. The reason for this is that even on an octacore hyperthreaded processor the OS is usually juggling several hundreds of processes and threads, sometimes thousands or more. The OS makes all that multitasking work by scheduling all of them to get a slice of CPU time one after another, meaning they get 'a few milliseconds of time at most to do their thing'.
Implicity this means that if you set a timeout for 1000 ms, chances are far from small that the current browser process won't even be running at that point in time, so it's perfectly normal for the browser not to notice until 1005, 1010 or even 1050 milliseconds that it should be executing the given callback.
Usually this is not a problem, it happens, and it's rarely of utmost importance. If it is, all operating systems supply kernel level timers that are far more precise than 1 ms, and allow a developer to execute code at precisely the correct point in time. JavaScript however, as a heavily sandboxed environment, doesn't have access to kernel objects like that, and browsers refrain from using them since it could theoretically allow someone to attack the OS stability from inside a web page, by carefully constructing code that starves other threads by swamping it with a lot of dangerous timers.
As for why the test yields 980 I'm not sure - that would depend on exactly which browser you're using and which JavaScript engine. I can however fully understand if the browser just manually corrects a bit downwards for system load and/or speed, ensuring that "on average the delay is still about the correct time" - it would make a lot of sense from the sandboxing principle to just approximate the amount of time required without potentially burdening the rest of the system.

Someone please correct me if I am misinterpreting this information:
According to a post from John Resig regarding the inaccuracy of performance tests across platforms (emphasis mine)
With the system times constantly being rounded down to the last queried time (each about 15 ms apart) the quality of performance results is seriously compromised.
So there is up to a 15 ms fudge on either end when comparing to the system time.

I had a similar experience.
I was using something like this:
var iMillSecondsTillNextWholeSecond = (1000 - (new Date().getTime() % 1000));
setTimeout(function ()
{
CountDownClock(ElementID, RelativeTime);
}, iMillSecondsTillNextWholeSecond);//Wait until the next whole second to start.
I noticed it would Skip a Second every couple Seconds, sometimes it would go for longer.
However, I'd still catch it Skipping after 10 or 20 Seconds and it just looked rickety.
I thought, "Maybe the Timeout is too slow or waiting for something else?".
Then I realized, "Maybe it's too fast, and the Timers the Browser is managing are off by a few Milliseconds?"
After adding +1 MilliSeconds to my Variable I only saw it skip once.
I ended up adding +50ms, just to be on the safe side.
var iMillSecondsTillNextWholeSecond = (1000 - (new Date().getTime() % 1000) + 50);
I know, it's a bit hacky, but my Timer is running smooth now. :)

Javascript has a way of dealing with exact time frames. Here’s one approach:
You could just save a Date.now when you start to wait, and create an interval with a low ms update frame, and calculate the difference between the dates.
Example:
const startDate = Date.now()
setInterval(() => {
const currentDate = Date.now()
if (currentDate - startDate === 1000 {
// it was a second
clearInterval()
return
}
// it was not a second
}, 50)

The javascript timing resolution in my browsers seems to be ~8ms. How can I increase it? [duplicate]

Something that has always bugged me is how unpredictable the setTimeout() method in Javascript is.
In my experience, the timer is horribly inaccurate in a lot of situations. By inaccurate, I mean the actual delay time seems to vary by 250-500ms more or less. Although this isn't a huge amount of time, when using it to hide/show UI elements the time can be visibly noticeable.
Are there any tricks that can be done to ensure that setTimeout() performs accurately (without resorting to an external API) or is this a lost cause?

Are there any tricks that can be done
to ensure that setTimeout() performs
accurately (without resorting to an
external API) or is this a lost cause?
No and no. You're not going to get anything close to a perfectly accurate timer with setTimeout() - browsers aren't set up for that. However, you don't need to rely on it for timing things either. Most animation libraries figured this out years ago: you set up a callback with setTimeout(), but determine what needs to be done based on the value of (new Date()).milliseconds (or equivalent). This allows you to take advantage of more reliable timer support in newer browsers, while still behaving appropriately on older browsers.
It also allows you to avoid using too many timers! This is important: each timer is a callback. Each callback executes JS code. While JS code is executing, browser events - including other callbacks - are delayed or dropped. When the callback finishes, additional callbacks must compete with other browser events for a chance to execute. Therefore, one timer that handles all pending tasks for that interval will perform better than two timers with coinciding intervals, and (for short timeouts) better than two timers with overlapping timeouts!
Summary: stop using setTimeout() to implement "one timer / one task" designs, and use the real-time clock to smooth out UI actions.

.
REF; http://www.sitepoint.com/creating-accurate-timers-in-javascript/
This site bailed me out on a major scale.
You can use the system clock to compensate for timer inaccuracy. If you run a timing function as a series of setTimeout calls — each instance calling the next — then all you have to do to keep it accurate is work out exactly how inaccurate it is, and subtract that difference from the next iteration:
var start = new Date().getTime(),
time = 0,
elapsed = '0.0';
function instance()
{
time += 100;
elapsed = Math.floor(time / 100) / 10;
if(Math.round(elapsed) == elapsed) { elapsed += '.0'; }
document.title = elapsed;
var diff = (new Date().getTime() - start) - time;
window.setTimeout(instance, (100 - diff));
}
window.setTimeout(instance, 100);
This method will minimize drift and reduce the inaccuracies by more than 90%.
It fixed my issues, hope it helps

I had a similar problem not long ago and came up with an approach which combines requestAnimationFrame with performance.now() which works very effectively.
Im now able to make timers accurate to approx 12 decimal places:
window.performance = window.performance || {};
performance.now = (function() {
return performance.now ||
performance.mozNow ||
performance.msNow ||
performance.oNow ||
performance.webkitNow ||
function() {
//Doh! Crap browser!
return new Date().getTime();
};
})();
http://jsfiddle.net/CGWGreen/9pg9L/

If you need to get an accurate callback on a given interval, this gist may help you:
https://gist.github.com/1185904
function interval(duration, fn){
var _this = this
this.baseline = undefined
this.run = function(){
if(_this.baseline === undefined){
_this.baseline = new Date().getTime()
}
fn()
var end = new Date().getTime()
_this.baseline += duration
var nextTick = duration - (end - _this.baseline)
if(nextTick<0){
nextTick = 0
}
_this.timer = setTimeout(function(){
_this.run(end)
}, nextTick)
}
this.stop = function(){
clearTimeout(_this.timer)
}
}

shog9's answer is pretty much what I'd say, although I'd add the following about UI animation/events:
If you've got a box that's supposed to slide onto the screen, expand downwards, then fade in its contents, don't try to make all three events separate with delays timed to make them fire one after another - use callbacks, so once the first event is done sliding it calls the expander, once that's done it calls the fader. jQuery can do it easily, and I'm sure other libraries can as well.

If you're using setTimeout() to yield quickly to the browser so it's UI thread can catch up with any tasks it needs to do (such as updating a tab, or to not show the Long Running Script dialog), there is a new API called Efficient Script Yielding, aka, setImmediate() that may work a bit better for you.
setImmediate() operates very similarly to setTimeout(), yet it may run immediately if the browser has nothing else to do. In many situations where you are using setTimeout(..., 16) or setTimeout(..., 4) or setTimeout(..., 0) (i.e. you want the browser to run any outstanding UI thread tasks and not show a Long Running Script dialog), you can simply replace your setTimeout() with setImmediate(), dropping the second (millisecond) argument.
The difference with setImmediate() is that it is basically a yield; if the browser has sometime to do on the UI thread (e.g., update a tab), it will do so before returning to your callback. However, if the browser is already all caught up with its work, the callback specified in setImmediate() will essentially run without delay.
Unfortunately it is only currently supported in IE9+, as there is some push back from the other browser vendors.
There is a good polyfill available though, if you want to use it and hope the other browsers implement it at some point.
If you are using setTimeout() for animation, requestAnimationFrame is your best bet as your code will run in-sync with the monitor's refresh rate.
If you are using setTimeout() on a slower cadence, e.g. once every 300 milliseconds, you could use a solution similar to what user1213320 suggests, where you monitor how long it was from the last timestamp your timer ran and compensate for any delay. One improvement is that you could use the new High Resolution Time interface (aka window.performance.now()) instead of Date.now() to get greater-than-millisecond resolution for the current time.

You need to "creep up" on the target time. Some trial and error will be necessary but in essence.
Set a timeout to complete arround 100ms before the required time
make the timeout handler function like this:
calculate_remaining_time
if remaining_time > 20ms // maybe as much as 50
re-queue the handler for 10ms time
else
{
while( remaining_time > 0 ) calculate_remaining_time;
do_your_thing();
re-queue the handler for 100ms before the next required time
}
But your while loop can still get interrupted by other processes so it's still not perfect.

Here's an example demoing Shog9's suggestion. This fills a jquery progress bar smoothly over 6 seconds, then redirects to a different page once it's filled:
var TOTAL_SEC = 6;
var FRAMES_PER_SEC = 60;
var percent = 0;
var startTime = new Date().getTime();
setTimeout(updateProgress, 1000 / FRAMES_PER_SEC);
function updateProgress() {
var currentTime = new Date().getTime();
// 1000 to convert to milliseconds, and 100 to convert to percentage
percent = (currentTime - startTime) / (TOTAL_SEC * 1000) * 100;
$("#progressbar").progressbar({ value: percent });
if (percent >= 100) {
window.location = "newLocation.html";
} else {
setTimeout(updateProgress, 1000 / FRAMES_PER_SEC);
}
}

This is a timer I made for a music project of mine which does this thing. Timer that is accurate on all devices.
var Timer = function(){
var framebuffer = 0,
var msSinceInitialized = 0,
var timer = this;
var timeAtLastInterval = new Date().getTime();
setInterval(function(){
var frametime = new Date().getTime();
var timeElapsed = frametime - timeAtLastInterval;
msSinceInitialized += timeElapsed;
timeAtLastInterval = frametime;
},1);
this.setInterval = function(callback,timeout,arguments) {
var timeStarted = msSinceInitialized;
var interval = setInterval(function(){
var totaltimepassed = msSinceInitialized - timeStarted;
if (totaltimepassed >= timeout) {
callback(arguments);
timeStarted = msSinceInitialized;
}
},1);
return interval;
}
}
var timer = new Timer();
timer.setInterval(function(){console.log("This timer will not drift."),1000}

Hate to say it, but I don't think there is a way to alleviate this. I do think that it depends on the client system, though, so a faster javascript engine or machine may make it slightly more accurate.

To my experience it is lost effort, even as the smallest reasonable amount of time I ever recognized js act in is around 32-33 ms. ...

There is definitely a limitation here. To give you some perspective, the Chrome browser Google just released is fast enough that it can execute setTimeout(function() {}, 0) in 15-20 ms whereas older Javascript engines took hundreds of milliseconds to execute that function. Although setTimeout uses milliseconds, no javascript virtual machine at this point in time can execute code with that precision.

Dan, from my experience (that includes implementation of SMIL2.1 language in JavaScript, where time management is in subject) I can assure you that you actually never need high precision of setTimeout or setInterval.
What does however matter is the order in which setTimeout/setInterval gets executed when queued - and that always works perfectly.

JavaScript timeouts have a defacto limit of 10-15ms (I'm not sure what you're doing to get 200ms, unless you're doing 185ms of actual js execution). This is due to windows having a standard timer resolution of 15ms, the only way to do better is to use Windows' higher resolution timers which is a system wide setting so can screw with other applications on the system and also chews battery life (Chrome has a bug from Intel on this issue).
The defacto standard of 10-15ms is due to people using 0ms timeouts on websites but then coding in a way that assumes that assumes a 10-15ms timeout (eg. js games which assume 60fps but ask 0ms/frame with no delta logic so the game/site/animation goes a few orders of magnitude faster than intended). To account for that, even on platforms that don't have windows' timer problems, the browsers limit timer resolution to 10ms.

Here are what I use. Since it's JavaScript, I will post both my Frontend and node.js solutions:
For both, I use the same decimal rounding function that I highly recommend you keep at arms length because reasons:
const round = (places, number) => +(Math.round(number + `e+${places}`) + `e-${places}`)
places - Number of decimal places at which to round, this should be safe and should avoid any issues with floats (some numbers like 1.0000000000005~ can be problematic). I Spent time researching the best way to round decimals provided by high-resolution timers converted to milliseconds.
that + symbol - It is a unary operator that converts an operand into a number, virtually identical to Number()
Browser
const start = performance.now()
// I wonder how long this comment takes to parse
const end = performance.now()
const result = (end - start) + ' ms'
const adjusted = round(2, result) // see above rounding function
node.js
// Start timer
const startTimer = () => process.hrtime()
// End timer
const endTimer = (time) => {
const diff = process.hrtime(time)
const NS_PER_SEC = 1e9
const result = (diff[0] * NS_PER_SEC + diff[1])
const elapsed = Math.round((result * 0.0000010))
return elapsed
}
// This end timer converts the number from nanoseconds into milliseconds;
// you can find the nanosecond version if you need some seriously high-resolution timers.
const start = startTimer()
// I wonder how long this comment takes to parse
const end = endTimer(start)
console.log(end + ' ms')

You could consider using the html5 webaudio clock which uses the system time for better accuracy

Stopwatch not working, it's going way too faster

As I was looking for a simple Stopwatch implementation in JS, I found this code http://codepen.io/_Billy_Brown/pen/dbJeh/
The problem is that it's not working fine, the clock go way too fast. I got 30 seconds on the screen when i got only 23 seconds on my watch.
And I don't understand why. The timer function is called every millisecond and should be updating the time correctly.
setInterval(this.timer, 1);
Is the problem coming from the browser or from the JS code.
Thanks in advance.

The timers in Javascript doesn't have millisecond precision.
There is a minimum time for the interval, which differs depending on the browser and browser version. Typical minimums are 4 ms for recent browsers and 10 ms for a little older browsers.
Also, you can't rely on the callback being called at exact the time that you specify. Javascript is single threaded, which means that if some other code is running when the timer triggers a tick, it has to wait until that other code finishes.

In fact the code you gave is imitating time flow, but it is not synchronized with system time.
Every millisecond it just invokes the function this.time, which performs recounting of millis, seconds and so on
without getting native system time, but just adding 1 to variable representing "imaginary milliseconds".
So we can say that resulting pseudo-time you see depends on your CPU, browser and who knows what else.
On our modern fantastically fast computers the body of this.time function is being executed faster than millisecond (wondering what would happen on Pentium 2 with IE5 on board).
Anyhow there is no chance for the this.time to be executed exactly in particular fixed period on all computers and browsers.
The simplest correct way to show the time passed since startpoint according to the system time is:
<body>
<script>
var startTime = new Date() // assume this initialization to be start point
function getTimeSinceStart()
{
var millisSinceStart = new Date() - startTime
, millis = millisSinceStart % 1000
, seconds = Math.floor(millisSinceStart / 1000)
return [seconds, millis].join( ':' )
}
(function loop()
{
document.title = getTimeSinceStart() // look on the top of page
setTimeout( loop, 10 )
}())
</script>
</body>
P.S. What #Guffa says in his answer is correct (generally for js in browsers), but in this case it does not matter and not affect the problem

can setInterval drift over time?

I have 2 node.js webservers. I cache data inside webservers. I sync the cache load/clear based on system time. I have done time sync of all my hosts.
Now I clear cache every 15 mins using following code:
millisTillNexthour = "Calculate millis remaining until next hour"
setTimeout(function() {
setInterval(function() {
cache.clear();
}, 60000*15);
}, millisTillNexthour);
My expectation is even if this process runs for ever, cache will be cleared every 15th minute of each hour of the day.
My question is: can setInterval drift over time?
For eg: right now it clears cache at 10:00 10:15 10:30 10:45 11:00 ......
Can it happen that instead of 10:15 system time, setInterval gets executed at 10:20 system time when it was supposed to clear cache at 10:15??
I am not sure how this works. Please shed some light. I hope I explained my question well.

I'm probably more than a bit late to the party here, but this is how I solved this particular time-slipping problem just now, using a recursively called setTimeout() function instead of using setInterval().
var interval = 5000;
var adjustedInterval = interval;
var expectedCycleTime = 0;
function runAtInterval(){
// get timestamp at very start of function call
var now = Date.now();
// log with time to show interval
console.log(new Date().toISOString().replace(/T/, ' ').replace(/Z/, '') + " runAtInterval()");
// set next expectedCycleTime and adjustedInterval
if (expectedCycleTime == 0){
expectedCycleTime = now + interval;
}
else {
adjustedInterval = interval - (now - expectedCycleTime);
expectedCycleTime += interval;
}
// function calls itself after delay of adjustedInterval
setTimeout(function () {
runAtInterval();
}, adjustedInterval);
}
On each iteration, the function checks the actual execution time against the previously calculated expected time, and then deducts the difference from 'interval' to produce 'adjustedInterval'. This difference may be positive or negative, and the results show that actual execution times tend to oscillate around the 'true' value +/- ~5ms.
Either way, if you've got a task that is executing once a minute, and you run it for an entire day, using this function you can expect that - for the entire day - every single hour will have had 60 iterations happen. You won't have that occasional hour where you only got 59 results because eventually an entire minute had slipped.

setInterval is definitely drifting (although I agree that it should not be). I'm running a Node.js server with an interval of 30 seconds. On each tick, a few async web requests are made which from beginning to end take roughly 1 second. No other user-level/application processing happens in the intervening 29 seconds.
However, I notice from my server logs that over the course of 30 minutes, a drift of 100ms occurs. Of course, the underlying operating system is not to blame for the drift and it can only be some defect of Node.js's design or implementation.

I am very disappointed to notice that there is a bug in the NodeJS implementation of setInterval. Please take a look at here:
https://github.com/nodejs/node/issues/7346#issuecomment-300432730

You can use Date() object to set specific time and then add a certain number of milliseconds to the date.

It definitly can because of how Javascript works (See Event Loop)
Javascript event loop executes the setInterval queue when other queued events are finished. These events will take some time and it will effect your setInterval function's execute time and it will eventually drift away as time passes.

setInterval should not drift in a perfect world. It might be delayed due to other things taking up system resources. If you need a more precise solution to what you have, use the clock() function to " calibrate " your nodes.

Getting certain frequency with setInterval method

In a javascript code I develop, some function should be called every 1 second. But to be sure that this operation takes place every 1 second, the following code is utilized:
setInterval(doIt, 500);
function doIt() {
var now = (new Date()).getTime();
if(lastUpdate + 1000 >= now) {
/// code...
lastUpdate = now;
}
}
As far as I know setInterval(doIt, 1000) doesn't always mean that it's called every one second.
Is the above solution is a valid one? If not, what do you recommend?

You could use setTimeout instead of setInterval, and make dynamic adjustments each time your function is called. The idea is to set the timeout for a number of milliseconds sufficient to carry you to the next second boundary.
function timeoutFunc() {
// do interesting things
var time = new Date().getTime();
setTimeout(timeoutFunc, 1000 - time % 1000);
}
You'd start it off with:
setTimeout(timeoutFunc, 1000 - new Date().getTime() % 1000);
Synchronizing with the server seems like a bad idea, because you have no way of knowing whether the client clock is synchronized to anything (like the NTP server network). If it's not, then you server synchronizations are going to make things look wrong at the client, because the client clock will always be what seems right.

well setInterval IS defined in milliseconds. so it means its called every X millisdconds.
however the system can freeze or something like that!
but theres no practical better solution, you approach is fine.
if you really have an extensive javascript client application the results could stretch a little bit.
a possible solution for that is to get the system time and have a counter in your function. then ever X executions you align with the system clock, calculate how many function calls you should have until now and speed up the interval or slow it down.
this is as far as you can get to perfection. but it will be only a matter of milliseconds and probably not worth the effort.
may i ask what you are developing?