In order for this property to work, the server has to send the information to the browser. which is widely supported by the browsers nowadays.
When a server does not send a lastModified date information to the browser, Netscape and Opera give a last modification date of 01/01/1970 at 00:00:00 hrs, while Internet Explorer gives today. So it is always a good practice to test the server if it is sending the necessary information to the browser before using this script.
outputs: 18/01/2008 16:43:32