dynamically position a window

[admin]

I am opening a window to display a calendar so users can select a date and it populates a field on the (underlying) page. How can I open the calendar window so it is always near the link that called it but not obscuring the link? Thanks for any help. (On some pages the link opening the calendar window might be at the top left of the page - on other pages it is at the bottom right of the page - sometimes it is in the middle. I need to make sure the calendar window always opens so it does not obscure the link that opened it.)