Javascript runs on the browser, ColdFusion on the server. Hence, Javascript variables are not available to ColdFusion.
You are apparently aware of one way of passing the variable to ColdFusion, namely, by submitting a form. The variable is then available on the form's action page as form.txt_expiration.
However, I was wondering why don't just do the obvious. Get the value of txt_expiration directly from the query!