Get last date for x day of a given month in Javascript

Question

I've been trying to figure out how to get the date of the last x day of a month in JavaScript without using a loop. I'm sure its to do with using the modulus operator but I cant quite get the formula right :-(

So for example the date for the last Monday in this month (06/2013) would be the 24th.

Can anyone shed some light on this? It's driving me nuts!

Thanks in advance.

Edit

I think I have it working now thanks to having a look inside how date.js do it, I was close with what I had though!

function getLastDay(year, month, dayOfWeek)
{
    var d = new Date(year, month, 0);
    var diff = (dayOfWeek - d.getDay() + 7 * -1) % 7;

    return new Date(year, month-1, d.getDate() + diff);
}

// Last Monday in for January 2013
getLastDay(2013, 1, 1); // Mon Jan 28 2013 00:00:00 GMT+0000 (GMT Standard Time)

Answer 1

Sorry, misread your question at first. Editing my response.

If you're willing to use a library, try date.js.

You can use the moveToNthOccurence with -1 to get the last occurence of a day in the month.

So:

Date.today().moveToNthOccurrence(1, -1)

should give you the last Monday of this month.