using instance variable within JavaScript function loaded in a partial

Im wondering, how do I go about allowing my javascript function to
load the updated instance variable that is loaded within a partial?
To be specific, I have a javascript function call:
onmousedown="changeTab(this)" for a tab in my layout. This function
passes an instance variable with an Ajax.Request like so:

changeTab(tab) {
new Ajax.Request( '/test/ajax', {parameters: { date: <%=
array_or_string_for_javascript(@date) %> })

@date is changed each time a different link_to_remote is called and
the link_to_remote catches this change and send the updated @date
variable, but my javascript function changeTab does not pass the
updated @date variable. How do I get the changeTab javascript
function to pass the new updated @date variable?

This code is only evaluated once so what happens is normal. One way of
doing things is to have the date be a parameter of changeTab (assuming
that whatever calls changeTab is updated after these requests).
Another is to have your requests update some global javascript