After add days using setDate, month does no change
===

I have some trouble with data manipulation in Javascript

I have one variable which is `firstDay`, that's date that I'm operating on.

To `firstDay` I add `X` days from one variable, and `Y` days from another:

    var from_date = new Date(firstDay); // firstDay is a string with date: 2012/07/28
    var to_date = new Date(firstDay);

    var X = 1;
    var Y = 5;
											
    from_date.setDate(from_date.getDate() + X);
    to_date.setDate(to_date.getDate() + Y);

So `from_date` now contains date 29.07.2012 and to_date is 02.06.2012. I don't know why date changes propetly, but months stay the same.

Where I make something wrong?