I just ran across a usage of Date.now() in a web application and noticed it throwing an error in IE8, our lowest common denominator of a browser. Here is a quick fix to make sure you get your number value back in all browser.

The now() method returns the milliseconds elapsed since 1 January 1970 00:00:00 UTC up until now as a Number.

Minimum Browser for Date.now()

Browser Version
Chrome 5
Firefox (Gecko) 3
Internet Explorer 9
Opera 10.50
Safari 4

Solid Tip: Date.now() is much faster than old methods (but doesn’t work before IE9). Compare them with this performance test.

Proper Method for Older Browsers

new Date().getTime();

If You Want Date.now() as a Function

Mozilla suggests the following shim with an expression to solve your issues.

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

If you end up adding the function yourself so that it is available, be sure to put the date into a variable so it doesn’t keep having to be instantiated. It will provide ~20% faster performance.