Poll server for file date?

Poll server for file date?

Hi.  I have a page that serves up data that is refreshed every 30 minutes.  I'd like to do the following:

Data last updated April 15, 2011 at 4:36 PM (16 minutes ago).

I'd like to refresh the date automatically without the user hitting Refresh.

Two questions.  One:  How do I poll the server every 30 seconds and check a file date?

Two:  How do I do the "16 minutes ago" part? 

Thanks for any help.