I started by looking at the lines of information in the cookies.txt file where Netscape keeps its cookies. The file had hundreds of cookies from sites I had visited recently, but was missing several sites that I visit infrequently. Then I checked the folder where Microsoft Internet Explorer keeps its cookies and I found only a modest number. It was clear that my batch of good cookies had, indeed, gone bad. There are two main reasons why cookies disappear, but web managers have control over only one of them.
Cookies contain an expiration date that tells the browser software when to delete the cookie.
Cookies without an expiration date last only until the user quits out of their browser software, so developers normally use a distant date when developing web software. Several years ago when developers started using cookies, it was common to use the example on the Netscape site as a guide. The other problem is one that is harder for web marketers to manage, which is the maximum number of cookies that browser software will store. Netscape's browsers, for example, keep only 300 cookies on file. As more cookies are received from sites, the oldest cookies get shoved out of the cookie jar no matter their expiration date.