Javascript Date.UTC() function is off by a month?

I was playing around with Javascript creating a simple countdown clock when I came across this strange behavior:

var a = new Date(), 
now = a.getTime(),
then = Date.UTC(2009,10,31),
diff = then - now,
daysleft = parseInt(diff/(24*60*60*1000));
console.log(daysleft );

The days left is off by 30 days.

What is wrong with this code?

Edit: I changed the variable names to make it more clear.

Answer

The month is zero-based for JavaScript.

Days and years are one-based.

Go figure.

UPDATE

The reason this is so, from the creator of JavaScript, is

JS had to “look like Java” only less so, be Java’s dumb kid brother or boy-hostage sidekick. Plus, I had to be done in ten days or something worse than JS would have happened.

http://www.jwz.org/blog/2010/10/every-day-i-learn-something-new-and-stupid/#comment-1021