I have a div id called modalpage
and have css. I need a javascript function which can dynamically shows popup for 20 mins and change in every 30 secs right now i have the following javascript function. Can anybody help me please

<script language="javascript" type="text/javascript">
function revealModal(divID)
{
    window.onscroll = function () { 
        document.getElementById(divID).style.top = document.body.scrollTop;
    };
    document.getElementById(divID).style.display = "block";
    document.getElementById(divID).style.top = document.body.scrollTop;
}

which is called by a input id button.

<input id="Button1" type="button" value="Click here" onclick="revealModal('modalPage')" />

Thanks