Event.timeStamp

The timeStamp read-only property of the Event interface returns the time (in milliseconds) at which the event was created.

time = event.timeStamp;

This value is the number of milliseconds elapsed from the beginning of the current document's lifetime till the event was created.

In newer implementations, the value is a DOMHighResTimeStamp accurate to 5 microseconds (0.005 ms). In older implementations, the value is a DOMTimeStamp, accurate to a millisecond.

<p>
  Focus this iframe and press any key to get the
  current timestamp for the keypress event.
</p>
<p>timeStamp: <span id="time">-</span></p>

function getTime(event) {
  var time = document.getElementById("time");
  time.firstChild.nodeValue = event.timeStamp;
}
document.body.addEventListener("keypress", getTime);

To offer protection against timing attacks and fingerprinting, the precision of Event.timeStamp 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
event.timeStamp;
// 1519211809934
// 1519211810362
// 1519211811670
// ...

// reduced time precision with `privacy.resistFingerprinting` enabled
event.timeStamp;
// 1519129853500
// 1519129858900
// 1519129864400
// ...

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