None.

A number representing the timestamp, in milliseconds, of the current time.

To offer protection against timing attacks and fingerprinting, the precision of Date.now() 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.

// reduced time precision (2ms) in Firefox 60
Date.now();
// 1519211809934
// 1519211810362
// 1519211811670
// …

// reduced time precision with `privacy.resistFingerprinting` enabled
Date.now();
// 1519129853500
// 1519129858900
// 1519129864400
// …

You can use Date.now() to get the current time in milliseconds, then subtract a previous time to find out how much time elapsed between the two calls.

const start = Date.now();
doSomeLongRunningProcess();
console.log(`Time elapsed: ${Date.now() - start} ms`);

For more complex scenarios, you may want to use the performance API instead.

• Polyfill of Date.now in core-js
• Performance.now()
• console.time()
• console.timeEnd()