Javascript setMonth() bug - Why is the date not working?

Recently I was doing some javascript date checking and recently encounted bugs that only occurred towards the end of the month. Today being the 31st May 2011 was the key to the problem.

var arrivalDate = new Date(); 
arrivalDate.setFullYear( $('#arrival-year').val() );
arrivalDate.setMonth( $('#arrival-month').val() -1 );
arrivalDate.setDate( $('#arrival-day').val() );

So in the example of this code today is 31 May 2011 and that is declared as the arrivalDate to begin with. Say I was setting the arrival date to 1 August 2011 in the order of the code:

• Starts as: 31 May 2011
• Sets the year: 2011 => 31 May 2011
• Sets the month: August => 31 August 2011

Here is the problem: 31 August 2011 ??? Says the computer, that doesn't exists silly. There are only 30 days in August, I'm going to give you a valid date I can think of.

So in order to fix, I tried setting the date before the month, which in theory should work, but was proving buggy still. In the end my final code ended up declaring the date in the object initialisation. As follows:

var arrivalDate = new Date( $('#arrival-year').val(), $('#arrival-month').val() -1, $('#arrival-day').val() ); 

Hope this helps someone!