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.

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 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

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 might get rounded depending on browser settings. In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 2ms. You can also enable privacy.resistFingerprinting, in which case the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

For example, with reduced time precision, the result of animation.startTime will always be a multiple of 0.002, or a multiple of 0.1 (or privacy.resistFingerprinting.reduceTimerPrecision.microseconds) with privacy.resistFingerprinting enabled.

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

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

Specifications

Specification
Web Animations # dom-animation-starttime

Browser compatibility

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This page was last modified on ⁨Aug 27, 2025⁩ by MDN contributors.

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