How to fake time in javascript?

sinon.useFakeTimers accepts a timestamp (integer) as parameter, not a Date object.

Try with

clock = sinon.useFakeTimers(new Date(2011,9,1).getTime());
new Date(); //=> return the fake Date 'Sat Oct 01 2011 00:00:00'

clock.restore();
new Date(); //=> will return the real time again (now)

If you use anything like setTimeout, make sure you read the docs because the useFakeTimers will disrupt the expected behavior of that code.