Just to preempt certain comments, I realize that many people have known this for ages, but there are many who have not, and to them this might just make their day.
Since ecmascript 5, Date.now() is the standard way of doing this. Generally speaking, I'd prefer to do that (with shims for older browsers) than do the equivalent of creating a new Date object and calling .valueOf on it (which is what +new Date does).
Also, "+new Date" is neat