Get the time on onload event of page

Hi All,
       I am doing auditing trails of an web application. I want the period of time of a visitor i.e for how long user stayed on that page.
      For that I can get the time of arrives on the page, but how to get the time of leaving that page, because user can leave that page by clicking another link OR by closing the browser OR by log out.
How to calculate the time?
Thanks