Date_select using instance variable

I’m trying to let users select a date, but I’m not storing the result in
a database, just in an instance variable. Is there a way to do this?
I’ve been trying many different variations. Here is my latest:

In the view I have:
<%= date_select (@params[:expiry], :expiry, :start_year =>
Date.today.year) %>

In the controller, nothing is passed into params[:expiry] for some
reason. How can I use date_select with an instance variable?

I’m guessing there’s an easy way to do this, anyone know how?