javascript date of last day of previous month

Question

Let's say I have 3 variables like this:

var Month = 8; // in reality, it's a parameter provided by some user input
var Year = 2011; // same here
var FirstDay = new Date(Year, Month, 1);

Now I want to have the value of the day before the first day of the month in a variable. I'm doing this:

var LastDayPrevMonth = (FirstDay.getDate() - 1);

It's not working as planned. What the right of doing it?

Thanks.

Answer 1

var LastDayPrevMonth = new Date(Year, Month, 0).getDate();

Answer 2

var LastDayPrevMonth = new Date(FirstDay);
LastDayPrevMonth.setHours(FirstDay.getHours()-24);