extend the expire time of a cookie

Hi, I have set the cookie to be expire after 24 hours, If people are not coming back then the cookie will be expired. But, I am having a funny questions, if people comes back at 23 hours 55 minutes later, and add something into their basket, then 5 minutes later, their shopping cart will be gone by then, becase the cookie is expired. Please tell me how to extend the expire time of the origional cookie if the client comes back within 24 hours. Thank you Mark