I'm guessing that you got this function from somewhere else?
The reason I say that, is because if you wrote it yourself, or even stopped and studied the code you would be able to identify what you need to change without the need to post on here.
I suggest you look at this line of code here:
(!chkdate.match(/^(0[1-9]|[12][0-9]|3[01])[\- \/.](?:(0[1-9]|1[012])[\-\/.](19|20)[0-9]{2})$/))
As this is the line that is doing the current comparison, all you need to do is move the "month" section to where the "day" section is and vice versa.