Problem when positioning a layer

Hi,
I have this small layer which has a monthly calendar for users to pick a date. Everything works just fine, the problem is that sometimes I need to have many date fields in a screen and the layer gets its position from the mouse position so it shows next to the button that calls it. The function is:
function showLayer(){
Layer1.style.visibility="visible";
Layer1.style.top=event.clientY+10;
Layer1.style.left=event.clientX-180;
}
The problem is that when you have to scroll down because the page gets longer, the layer shows way up. I know that this happens because x and y tell it the position of the screen but I would need to add the offset that has been scrolled. Is there a way to do this? how do I know the portion of screen that has been scrolled? If it helps, this is the code I used to create the layer.
<div id="Layer1" style="position:absolute; width:198px; height:190px;
z-index:1; left: 560px; top: 415px; visibility:hidden; background-color: #FFFFFF;
layer-background-color: #FFFFFF; border: 1px solid #000000; overflow: hidden">&nbsp;</div>
Thanks in advance.

You can use the tag, that gives you the total of the available screen, and then just calculate how far from the top of the screen the button is, and then position the button by the availabe screensize minus the absolute button position.

- Pheifel