Animation: startTime property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2020.

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See full compatibility

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 timeline 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).

Value

A floating-point number representing the start 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

Syncing different animations

In the following example, we can sync all new animated cats by giving them all the same startTime as the original running cat. Note that this is only possible with the Web Animation API: it is impossible to sync two separate animations with CSS animations.

<div id="css-cats">
  <h2>Cats animated with<br />CSS Animations</h2>
  <div class="cat with-css"></div>
  <button id="insert-css-cat">Add a Cat</button>
</div>

<div id="waapi-cats">
  <h2>Cats animated with the<br />Web Animation API</h2>
  <div class="cat" id="with-waapi"></div>
  <button id="insert-waapi-cat">Add a Cat</button>
</div>

css

/* All cats have the same dimensions and the same sprite for a background image. */
.cat {
  background: url("/shared-assets/images/examples/web-animations/cat_sprite.png") -600px
    0 no-repeat;
  height: 150px;
  width: 100%;
}

/* The cats animated with CSS have their running animations set with CSS */
.cat.with-css {
  animation: 0.75s steps(13, end) infinite run-cycle;
}

/*
  The keyframes for the CSS running animation.
  This moves the background image sprite around.
*/
@keyframes run-cycle {
  from {
    background-position: -600px 0;
  }
  to {
    background-position: -600px -1950px;
  }
}

#css-cats,
#waapi-cats {
  text-align: center;
  vertical-align: top;
  min-width: 300px;
}

body {
  background: #e5e6e9;
  color: #071933;
  font-family: sans-serif;
  display: flex;
  flex-wrap: wrap;
}

const cssCats = document.getElementById("css-cats");
const waapiCats = document.getElementById("waapi-cats");
const insertCSSCat = document.getElementById("insert-css-cat");
const insertWAAPICat = document.getElementById("insert-waapi-cat");

// The same information as @keyframes run-cycle
const keyframes = [
  { backgroundPosition: "-600px 0" },
  { backgroundPosition: "-600px -1950px" },
];
// The same information as .cat.with-css
const timing = {
  duration: 750,
  iterations: Infinity,
  easing: "steps(13, end)",
};

const catRunning = document
  .getElementById("with-waapi")
  .animate(keyframes, timing);

function createCat() {
  const newCat = document.createElement("div");
  newCat.classList.add("cat");
  return newCat;
}

insertCSSCat.addEventListener("click", () => {
  const newCat = createCat();
  newCat.classList.add("with-css");
  cssCats.insertBefore(newCat, insertCSSCat);
});

insertWAAPICat.addEventListener("click", () => {
  const newCat = createCat();
  const newAnimationPlayer = newCat.animate(keyframes, timing);
  // set start time to be the same as the original .cat#with-waapi
  newAnimationPlayer.startTime = catRunning.startTime;
  waapiCats.insertBefore(newCat, insertWAAPICat);
});

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of animation.startTime may be reduced depending on browser settings.

The value of this property can come from two sources: supplied by script, or calculated from AnimationTimeline.currentTime. The timeline's underlying clock can already be rounded before calculation; see the timeline's reduced time precision for its rounding intervals.

In Chrome, the browser does not apply additional timer rounding. In Safari, the browser rounds the returned value to 0.001 ms, the resolution used to represent animation times.

In Firefox, the browser rounds the returned value to 0.02 ms by default, including in cross-origin-isolated contexts. If privacy.resistFingerprinting is enabled, the rounding interval is 16.667 ms or the interval configured by privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

For example, these are possible values in Firefox:

// Reduced time precision (0.02 ms) with default settings
animation.startTime;
// Might be:
// 23.4
// 24.18
// 25.5
// …

// Reduced time precision with `privacy.resistFingerprinting` enabled
animation.startTime;
// Might be:
// 50.001
// 66.668
// 83.335
// …

Specifications

Specification
Web Animations # dom-animation-starttime

Browser compatibility

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This page was last modified on Sep 1, 2026 by MDN contributors.

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