Any way to force login cookies to expire?

If I fail to log out of “admin” on a particular computer, I have noticed that bringing up my webpage even a week later on the same computer “admin” is still signed in. I assume this is because of some cookie that is stored locally. Is there any way to force that cookie to expire, so that I do not have to worry that I may have forgotten to log off, thereby leaving my “admin” privileges up for grabs?