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).

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.

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:

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

/* A function that makes new cats. */
function addCat() {
  const 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
  const newCat = addCat();

  // animate said cat with the WAAPI's "animate" function
  const 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);
}

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 20 µs in Firefox 59; in 60 it will be 2 ms.

// 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.

• Web Animations API
• Animation
• Animation.currentTime for the current time of the animation.