Date.now()

The static Date.now() method returns the number of milliseconds elapsed since January 1, 1970 00:00:00 UTC.

Date.now()

A Number representing the milliseconds elapsed since the UNIX epoch.

This method was standardized in ECMA-262 5^th edition. Engines which have not been updated to support this method can work around the absence of this method using the following shim:

if (!Date.now) {
  Date.now = function now() {
    return new Date().getTime();
  };
}

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

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

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

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

• Performance.now() — provides timestamps with sub-millisecond resolution for use in measuring web page performance
• console.time() / console.timeEnd()