GetDay In Calendar - Code Only Works For Months With 30 Days In It

I am using the following script to have a calander displayed in my form. It displays a calender on which a user can select a date. If the date is a friday, something else should happen in the form than when selecting other days (see final 15 lines, no need to be specific).

The problem is that the code only works for months with 30 days in it. E.g. The september month works fine, when a friday is selected (javascript:setCalendarControlDate(2009,9,25)) the correct changes occur. For the October month (31 days) the same changes that occur correct in september occur in october when selecting thursday, but these should be when selecting friday instead! For November (30 days) everything works fine again, but then for December (31 days) the changes occur again at thursday instead of friday For February (2010) I need to select sunday to get the needed changes.

Like I said, I think it has something to do with the amount of days in a month. Since the code works fine on months with 30 days, but not on months with other amounts of days. Can anyone tell me what's wrong in the code?

function positionInfo(object)
{var p_elm = object;
this.getElementLeft = getElementLeft;
function getElementLeft()
{var x = 0;
var elm;
if(typeof(p_elm) == "object")
[Code].....