populating textbox

I have a pop up calendar that I use to populate txtBidOpenDate.Text. This works fine. What I'd like to do is populate a second textbox with a date that is seven days greater. I'm using the code below, which works fine. The problem is, I'd like the textbox to populate right away and not wait for a post back? Any Suggestions?

txtIntDisEndDate.Text = DateAdd(DateInterval.Day, 7, CDate(txtBidOpenDate.Text))

You could write a script and then in the 'onselectionchanged' property of the calendar call the function.

VB Code:

<script runat="server">     
sub Set_Date(sender as Object,e as EventArgs)       
txtIntDisEndDate.Text = DateAdd(DateInterval.Day, 7, CDate(txtBidOpenDate.Text))
end sub         
</script>

Then in the <asp:calendar

VB Code:

<asp:calendar onselectionchanged="Set_Date" id="calCh.......

Hope it helped
Cheers

what if the calendar is a different page?