need to auto update field when date expires in another column

need to auto update field when date expires in another column

First let me say i am a newbie to this application and to writing script. so please be explicit.

I have a field in my application (named expires_on) that holds an expiration date, I also have a field that is a check box (named active). The Active box is NOT checked by default and will return the answer of NO when checked. I only want text in the Active field that states NO.

I need script to make the check box automatically check itself when the expiration date has passed.

thanks