That is, the links are expired on an absolute time scale. One way is to keep track of when the user last visited our site, and make the comparison to that time stamp rather than to the file's creation or modification date. When a web browser makes a request to a server, the request is not connected to any previous or following request. No information about previous requests is passed along, and nothing we do in our request is saved for later ones. The best and easiest way is to use HTTP cookies, which nearly every browser supports.
Cookies are variables set by the server and stored on the client's computer.
Cookies allow us to track state across transactions by storing information on the user's computer. When the server next encounters the user, it can compare the time stamp on the cookie with the time stamp on the file. Each time the user visits our site, we set a cookie. The cookie's expiration date is set to be one week in the future, meaning that if the cookie exists, this user visited our site within the last week. That about does it for reading the cookie.