Unless the OP noted in the initial creation of the thread that the end time is subject to change, they can't just decide to change their rules after the auction ends or else they may receive negative feedback from users.
If you believe you will want to extend the time later on in your auction you need to give fair warning to your bidders by letting them know you may extend the time. If the OP doesn't say something along the lines of "Auctions times may be extended" then you should always stick with the scheduled end time. This warning should be posted BEFORE edits in the OP so as to not confuse your bidders. At the end time, the person with the highest bid is the winner.