Prepare for It. Like the situation above, you should go into the conversation knowing how you'll respond if your boss asks you to extend your notice. If you've landed another job offer and have already set a start date, this may not be possible__so practice sticking to your guns: "Because I have a new opportunity lined up, my last day here is firm.
However, at most companies, there's no formal requirement that you do so. Most employment in the United States is called "at will", which means that your current employer can fire you at any time and for any reason (except those that are prohibited under law).