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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The Animation.startTime property of the Animation interface is a double-precision floating-point value which indicates the scheduled time when an animation's playback should begin. 

An animation’s start time is the time value of its DocumentTimeline when its target KeyframeEffect is scheduled to begin playback. An animation’s start time is initially unresolved (meaning that it's null because it has no value).

Syntax

var animationStartedWhen = Animation.startTime;

Animation.startTime = newStartTime;

Value

A floating-point number representing the current time in milliseconds, or null if no time is set. You can read this value to determine what the start time is currently set at, and you can change this value to make the animation start at a different time.

Examples

In the Running on Web Animations API example, the we can sync all new animated cats by giving them all the same startTime as the original running cat:

var catRunning = document.getElementById ("withWAAPI").animate(keyframes, timing);

/* A function that makes new cats. */
function addCat(){
  var newCat = document.createElement("div");
  newCat.classList.add("cat");
  return newCat;
}

/* This is the function that adds a cat to the WAAPI column */
function animateNewCatWithWAAPI() {
  // make a new cat
  var newCat = addCat();

  // animate said cat with the WAAPI's "animate" function
  var newAnimationPlayer = newCat.animate(keyframes, timing);

  // set the animation's start time to be the same as the original .cat#withWAAPI
  newAnimationPlayer.startTime = catRunning.startTime;

  // Add the cat to the pile.
  WAAPICats.appendChild(newCat);
}

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of animation.startTime might get rounded depending on browser settings.
In Firefox, the privacy.reduceTimerPrecision  preference is enabled by default and defaults to 20us in Firefox 59; in 60 it will be 2ms.

// reduced time precision (2ms) in Firefox 60
animation.startTime;
// 23.404
// 24.192
// 25.514
// ...


// reduced time precision with `privacy.resistFingerprinting` enabled
animation.startTime;
// 49.8
// 50.6
// 51.7
// ...

In Firefox, you can also enabled privacy.resistFingerprinting, the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

Specifications

Specification Status Comment
Web Animations
The definition of 'Animation.startTime' in that specification.
Working Draft Editor's draft.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support39 ?

48

39 — 481

No26 No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support3939 ?

48

39 — 481

26 No4.0

1. From version 39 until version 48 (exclusive): this feature is behind the dom.animations-api.core.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

See also

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