Date.prototype.getTime()

const moonLanding = new Date("July 20, 69 20:17:40 GMT+00:00");

// Milliseconds since Jan 1, 1970, 00:00:00.000 GMT
console.log(moonLanding.getTime());
// Expected output: -14182940000

var birthday = new Date(1991, 9, 17);
var copy = new Date();
copy.setTime(birthday.getTime());

var end, start, i;

start = new Date();
for (i = 0; i < 1000; i++) {
  Math.sqrt(i);
}
end = new Date();

console.log("Operation took " + (end.getTime() - start.getTime()) + " msec");