Why this operation with date (number of days between 2 dates) return this value?

According to this question, I wrote "my code" (without Math.abs, I don't need it) :

var oneDay = 24 * 60 * 60 * 1000; // hours*minutes*seconds*milliseconds
var firstDate = new Date("2011", "09", "28"); // 28 september 2011
var secondDate = new Date("2011", "09", "30"); // 30 september 2011

var notti = ((secondDate.getTime() - firstDate.getTime()) / (oneDay));
if (notti < 1)
    notti = 1;
else
    notti = Math.round(notti);

alert(notti);

and it print 2 (correct).

Now, If I do this :

var oneDay = 24 * 60 * 60 * 1000; // hours*minutes*seconds*milliseconds
var firstDate = new Date("2011", "09", "28"); // 28 september 2011
var secondDate = new Date("2011", "10", "01"); // 01 october 2011

var notti = ((secondDate.getTime() - firstDate.getTime()) / (oneDay));
if (notti < 1)
    notti = 1;
else
    notti = Math.round(notti);

alert(notti);

it print 4. Why 4? It should be 3... Do you know about this problem?

The month argument in the date constructor (and other date methods) runs from [0.11] not [1..12] so:

new Date("2011", "09", "28"); // 28 september 2011

is actually Fri Oct 28, not September.

Javascript months are zero based. So October has 31 days.

new Date("2011", "9", "31"); // October 31st

Because...

new Date("2011", "09", "28").toString()

... returns:

Fri Oct 28 2011 00:00:00 GMT-0400 (EDT)

This is because JavaScript Data is based on the Java Date object, which is a mess. See also "Puizzle 61: The Dating Game" in the book JavaPuzzlers for an explanation.