Automatic Trigger Of Reminder In PHP?

My requirement was to generate a popup once you immediately login. windows.alert('please call') then the popup should come for every 2 hrs.

Even though you navigate to different pages on the site..This should still come..

I have started something like this:-- 1. Calling this script on everypage.

Code:

<script type="text/javascript">alert_settimer();</script><script type="text/javascript">
alert_settimer();

[Code]....

It is able to give me a popup for every 2 hrs. But whenever the user reloads the page after 1 hr. The function is called again and the count starts from 0hrs again. OR for example if 20 mins already passed then if i am going to a new page then the function is called again and so the timer starts again. I am not getting any idea how to restrict it calling every time. OR send that 1 hr again to the function so that it starts from there. I tried some thing like setting the value in a session variable and then starting the counter from that value.