Date.prototype.getTime()

A number representing the milliseconds elapsed between 1 January 1970 00:00:00 UTC and the given date.

To offer protection against timing attacks and fingerprinting, the precision of new Date().getTime() 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
new Date().getTime();
// 1519211809934
// 1519211810362
// 1519211811670
// ...

// reduced time precision with `privacy.resistFingerprinting` enabled
new Date().getTime();
// 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.

Constructing a date object with the identical time value.

// Since month is zero based, birthday will be January 10, 1995
var birthday = new Date(1994, 12, 10);
var copy = new Date();
copy.setTime(birthday.getTime());

Subtracting two subsequent getTime() calls on newly generated Date objects, give the time span between these two calls. This can be used to calculate the executing time of some operations. See also Date.now() to prevent instantiating unnecessary Date objects.

var end, start;

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

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

• Date.prototype.setTime()
• Date.prototype.valueOf()
• Date.now()