In order to access the file system (to find your posts and determine which are most recent) you will probably need some kind of server-side technology. Javascript (and of course HTML5) for example runs on the client's browser and is not touching your server's file-system.
You have to look at what kind of server-side technologies are available to you. For example, can you use PHP, ASP.net, etc? Take a look at where your site is hosted and see what you can find, do a little research, then edit your question accordingly.
Depending on how set you are on this feature this may be more trouble to set up than it's worth but in terms of difficulty, it is not hard to do, and it is a good way to learn the basics of programming.