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.

Solution 1:

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

Solution 2:

As Eric said, this is due to months being listed as 0-11 range.

This is a common behavior - same is true of Perl results from localtime(), and probably many other languages.

This is likely originally inherited from Unix's localtime() call. (do "man localtime")

The reason is that days/years are their own integers, while months (as a #) are indexes of an array, which in most languages - especially C where the underlying call is implemented on Unix - starts with 0.