performance.now and document.timeline.currentTime

MDN performance.now():

The performance.now() method returns a high resolution timestamp in milliseconds. It represents the time elapsed since Performance.timeOrigin

MDN document.timeline.currentTime

This timeline expresses the time in milliseconds since Performance.timeOrigin.

Web Animations:

Furthermore, since the time values of the default document timeline have a zero offset from the time origin, document.timeline.currentTime will roughly correspond to Performance.now()

console.log(performance.now(), document.timeline.currentTime)

has the different values. Why?

Because the timeline's currentTime is only updated when the Web Animations are, which is in the update the rendering steps of the event-loop.

So if you keep requesting currentTime during a loop, you'll get only one value:

const timestamps = new Set();
const t1 = performance.now();
while (performance.now() - t1 < 3000) { // perform below code for 3s
  timestamps.add(document.timeline.currentTime);
}
console.log([...timestamps]); // a single value

Note that this timestamp should be the same as the one passed to requestAnimationFrame callbacks in a Window context though:

let i = 0;
const anim = (timestamp) => {
  console.log('same timestamps', timestamp === document.timeline.currentTime);
  if (i++ < 10)
    requestAnimationFrame(anim);
}
requestAnimationFrame(anim);