To save time, browsers often load a frequently visited page from a local cache on the client’s computer rather than request it from the relevant Web server. The catch with this method is that a more up-to-date version of the page may exist on the server.
To get around this, you can set an expiration date for your page by including a tag between theand tags:
In this case, the expiry date is set to 9am on 9th August 1998 (note that the date has to be in the format shown here). The first time after this date that the browser tries to load the page, it will request it from the server instead of loading the outdated copy from the local cache.
This technique also provides a neat way to ensure that a page is always loaded from the server instead of the local cache. You can achieve this by including an illegal expiry date, which is usually done with a zero like this: